When using boats that travel through water and resurface if that same boat is to be submerged underwater, if it is submerge at an later time say even 10 minutes later the boat will remove the player and it make it unusable for a short period of time even if it is resurfaced. Once the boat let's the player get back on it will launch up like it was still under water and let the player use it again. This is a problem for my ice race track that has small water lifts that doesn't keep the boat under very long at all causing players who take the first water way shortcut to be removed from their boat the second water way shortcut they encounter.

Here's a link to the two videos showing that it only happens when you go through two water falls

https://1drv.ms/f/s!AmHWlleLm_9qgYIP3asP8jALFdIexg?e=SknsAL

Minecraftwaterbug