lightning striking water, makes an appreciable water wave?

Perhaps the heat ripping through the air (causing that thunder snap) could create sound waves that transition into water waves too?

love that delayed scream.

but I agree with Greg. good likelihood of ripples, caused by heating and possibly the shockwave.

Actually, it's not a silly question at all. I never really gave it any thought until you brought it up. It would be interesting to know what would happen if lightning struck a still body of water. While, I really don't believe the transfer of mass from moving electrons would have any effect, as I mentioned earlier, the effect of heating and as TLW mentioned, the sonic shockwave probably would. Another force that may cause a motion of water could be due to the electrostatic forces due to the high voltages (rather than strike current) involved. It is said that before a lightning strike to ground, electrostatic forces can cause your hair to stand on end. I wonder if these same forces could cause motion in still water preceding the strike.

I've seen estimates of an average lightning strike having that much energy (and wattage) but some say that's a little high. Regardless, keep in mind that all that power is distributed over the entire length of the lightning bolt. Often it's a number of miles long. The power is also dissipated in a variety of ways. Some as light, some as heat, and a lot as RF and even X-Ray radiation. Only a small fraction is actually dissipated at, or close to, the point the bolt hits the water. It would therefore probably be possible to simulate an actual lightning strike over a short distance close to a body of water.

In fact, at one of the science museums, I think it was in Boston, I saw a lightning simulator that could produce an actual lightning strike but only about 10 feet in length. Something like that could be used to see the effect of lightning hitting a pool of water.

The impact of the electrons is negligible. Electrons don't have much mass and they don't move fast at all. The average electron speed in a high current rail (25kA) is a lot lower than you could run along the rail. So when you run past such a rail, you will be faster at the end of it as all electrons entering the rail at the starting point. The electric pulse is what travels with near light speed but not the electrons them self. Imagine a mile long train. When the train engine starts moving, it doesn't take long until the last car is moving. So the impulse of motion travels rather quickly through the entire train. But it takes a very long time until the last car has passed the starting point of the train engine. So the cars them self which represent the electrons travel real slow but the impulse passing somewhat delayed through the buffers of the cars is not. The buffers of the "electron train" in a wire is the electrostatic repulsion of like charges.
Don't confuse this with the "individual" speed of electrons. The direction of this high speed is random so the general direction of the sum is zero when no voltage is present and still rather low if a voltage is present.

In fact the first physical experiment that proved that electrons are what move "free" through a wire was using a huge cable drum with a very sensitive voltmeter attached to the ends of the wire. The drum was then rotated very fast and then stopped "in a crash". There was a very small voltage which was caused by the momentum of the electrons and the polarity revealed that the moving charges were in fact the electrons. And the energy from the momentum of the rotation was very small so they needed a very sensitive voltmeter and a huge drum in order to prove anything.

The reason for this experiment done over a 100 years ago was to find out the real direction of flow of electricity. We still consider electricity is traveling from plus to minus which is not true in most cases, it's the other way round. The reason for this point of view is that you simply can't see electrons but you can see ions. Running a current through a conductive solution lets you see ions flowing through the solution. And those travel from plus to minus. It took a long time for mankind to realize that electric charges flowing through wires don't do that the same way and the experiment I described above was the first hard evidence. When this question was finally settled, research on special conductors and insulation could be started which eventually lead to the discovery of the "imperfect junction effect" over the discovery of semiconductors to transistors and finally MOS technology and other microelectronic circuits.

So the impact of the electron flow wouldn't do much. The kinetic energy of the electrons of a lightning strike is lower than throwing the tip of a pin into the lake. That's why "ion jets" can work. You need to spit out electrons, too which cause an impulse into the opposite direction, but the mass difference between an ion and its missing electrons is so vast that the electrons don't matter at all.

As mentioned, heat is a very great factor here. The temperature of an electric arc inside the air is a lot greater than the surface temperature of the sun. At the point of impact, water will flash to steam which is similar to an explosion right beneath the water surface. But the energy density lowers to the cube with the radius of the semisphere the energy divides into all directions so the volume where the energy is great enough to flash water to steam is small.
A lightning has not that much energy. While the voltage and the current is mind boggling, the duration is less than 10µs. And energy is voltage * current * time. A lot of the energy is lost in the air in form of the light we can see, radiation we can't see or feel (e.g. plenty of X-rays) and mechanical energy we can hear as thunder. What works the ground in a "standard" lightning is just less than 10kWh. And your electric boiler or flash heater won't give you that much warm water for 10 "units of energy" your utility company charges you. That's nothing compared to a lake.

But there are very intense effects at work, the Lorenz force will apply a good whack to the water since the vertical lightning and the horizontal surface of the water is 2/3 of a railgun configuration with the water as the projectile. Not very effective and the short duration won't apply much energy, but the water will become accelerated by the massive current alone.

Actually, the opposite happens every couple of hundred lightning strikes in the clouds during a very rare so called "positive lightning". It has 8 times more energy than a standard lightning, wasn't discovered until the mid 1990s and blows some atmosphere into space due to the Lorenz force. From space, you can sometimes see mushroom clouds ("Red Sprites") blowing out of the top of thunderclouds accelerating some of the atmosphere above into space.

Until the existence of positive lightning was discovered, it had caused a lot of trouble in the science and even political world! The scientists wondered why 5x over-engineered lightning protected electric circuits were sometimes but rarely blown to bits anyway and cold war "listening posts" which can sense the infrasound of a nuclear explosion (which travels 5 times around the world by the way) had sometimes picked up sound patterns which looked like a nuclear test but were not very strong. So the east and the west suspected that their enemies were playing with miniature nuclear weapons. It had turned out that those sounds were produced by the rare positive lightnings. Long before end of the cold war, an engineer in one of those listening stations was able to determine that the time of one of their weird alarms was identical with a unusual hard lightning strike not that far away. He was ignored by his supervisors. It took a long time after the end of the cold war to match "Ivan's nuclear tests" with the data of the listening posts and proof that there really was something else which caused those alarms. The Columbia Space shuttle had an "Earth observation" packet on board on its very last mission to investigate positive lightnings. The cynic part is that some people belief that they have evidence that the Columbia disaster was caused by a "Megalightning strike" instead of some insulation foam. They think the Columbia was destroyed by something it was sent up to investigate.

Here's an interesting article where a positive lightning had blown a small aircraft into bits: www.damninteresting.com/the-power-of-positive-lightning/