Ok so, i tamed a hippocampus 3 times and i was trying to ride em underwater but when im already in the water it kick me off , i already reset the world and install all again but it keep kicking me off
Men, apparently in version 2.1.3 - 1.16.4 this bug does not happen, i tested it here on my minecraft and did not give this bug, it only happens in versions 2.1.4 and 2.1.5, so if you want to ride on your hippocampus go to version 2.1.3, until the developers fix this bug. I hope this helps.