I’m no expert on tsunamis but its my understanding that they are a fast moving wave caused by an earthquake that passes through the ocean and then results in a huge powerful wall of water that is created and breaks when it gets close to shore.
Say you are a scuba diver far out in the ocean and are underwater when a tsunami wave passes. What would the diver experience? Would it be noticed? Would the wave cause physical damage? What about boats and ship on the surface? What is the effect on them?
There’s video from a Japanese ship riding over a tsunami from several years ago. It’s clear there’s something major going on, but it’s not nearly as dramatic as you might expect.
The tsunami only becomes a wall of water as the wave starts dragging the bottom - the bottom slows the wave down and it’s energy pushes the water higher than normal. The same effect is seen with regular ocean waves, but with a tsunami it’s much more water being displaced by a much stronger force. As with normal waves, a tsunami pulls water out before coming ashore - just watch any beach cam. Also, a tsunami is not necessarily one wave, but essentially the surface of the ocean rising temporarily in a specific location, then retreating.
As already said, the answer to both is “nothing”. Tsunami waves are extremely fast-moving but also extremely low amplitude and have a very large wavelength (potentially up to hundreds of miles) until they hit shallow water. Only then do they become a dangerous, mighty force.
Based on that video - a scuba diver might notice going up and down a bit.
Like mentioned, tsunamis like regular waves start to crest (rise and go tubular, etc) when they hit shallow water. If you recall the videos of damage in the Japanese massive earthquake, those waves mostly did not even break in the harbours like the classic movie giant wall of water with foam on top. It was more like a series of successive surges, often aggravated by the surge coming into a progressively narrower bay, and flowing over the sea walls.
