You get a bonus action called "Tail Strike", which can be used when prone or grappled (main thing is that you can use this attack any time as long as your tail isn't chained or tied)

Melee attack, 1d6 bludgeoning, + STR or DEX on attack roll, and target becomes prone until end of your next turn when on a critical hit.

You have Darkvision with a range of 60 feet.

You can hold on and be carried by a willing medium creature, you have occupied their space and you go where the creature moves until you let go. You can't take actions, but you can still use your Tail Strike ability.

Being mounted on a creature that is one size larger or more than you applies the normal mounting rules where you can use weapons such as spears.

You can breath under-water and on land and not take on the effects of drowning, but suffocation from falling debris or choking via grappling is still possible as you have gills and lungs.