What does this death message mean?

I was playing on a survival multiplayer server, when suddenly the following message appeared:

The player seemed to be very confused (and mad), saying that he has no idea what killed him.

Just the phrase "fell out of the water" makes no sense. I've looked up the death messages on the Wiki, but all I found was the death message being listed in death messages caused by falling, which also makes no sense, since the player said he was in the ocean at the moment.

What could've caused this death?

Edit:

The player who this happened to said they were not even inside a boat, but swimming down to a ship wreck instead, when suddenly they lost all their HP.

This death message is a very rare one, that can be triggered by using a Trident with the "Riptide" enchantment. It is caused by riptiding (from under the ocean) to a high altitude, then riptiding downwards while falling back into the ocean. But he said that he was swimming down to a shipwreck, right? Which means that either there is another way to trigger this death message, or that he is lying.

This is the "official" way to recreate this death message:

1. Make sure you are fully submerged in the ocean and it is raining.
2. Using your Trident with the Riptide enchantment, riptide upwards.
3. Keep riptiding until you reach an extremely high altitude (like 5000 or 6000 blocks)
4. Then, while you fall back into the ocean, riptide downwards back into the ocean.
5. When you hit the ocean floor, this death message should appear.

Another theory is that a server glitch, or a troller, teleported him to a high altitude. Then as he fell, he riptided downwards and hit the ocean floor.

Or maybe the server intended it to show this death message upon doing something.

There are a lot of ways this death message can appear. These are just my theories.

Hope I helped...