How does it float? Do you have to maintain water depth by using spacebar or some other button?
I can only give out enough information to not spoil the fun, because intentions say he'd like to make a 'splash' with it.
But it's like, taking a plane into the water. Or in this case.. It's like Roblox, where you can have a vehicle go wherever and you just have to aim for it to go anywhere. It has depth difference, you can float up, or sink down, just like height control, and you can like 'fly' around underwater. It is able to stop and stay still without any problem. As for water and air, it has a way to reject being above the waterline, you can only go so far out before it rejects you.
