Water mounts will not allow you to descend until they've broken sea level for some reason. This causes them to ram your head into cave ceilings relentlessly if you are trying to remount to leave an underwater cave, causing frustrating and unnecessary deaths. Not sure if this behavior is intended or it's just a bug, but it's impossible to mount a water pokemon underwater and stay underwater.

Hi,

After testing this, I can confirm that upon dismounting a pokemon underwater, and getting back onto it, even holding the descend key doesn't actually do anything until the pokemon has popped out of the water. Once it has reached the surface, the keybind does then start to work again, but only after you've gone from the bottom of the ocean, all the way to the top first.

Thank you for the report.
I have marked the ticket so it can be sent to the devs.

Hi there! We're currently going through old tickets, so I've retested this issue to confirm that it's still an issue in the current version (8.3.6), and as such, I'll go ahead and move this to internal! Thank you!