Group swimming lesson or swim team game.
One player gets a kickboard puts one toy on top of it. Player then floats on back with kickboard balanced on their stomach like it is a turtle shell.
Goal is to get to the other side without toy falling off the kickboard, or his shell falling off.
Other players can do 1 of two things, depending on skill level. They can be boulders in the water and block the turtle from their goal, but cannot move.
They can pretend to be other turtles and without using their hands try and steal the toy and put it on top of their kickboard.
Other players can be sharks and after a head start, chase down the turtle, but can only tag their head.
Other players can create a current, or rapids by waving kickboards underwater or just at the surface to make it difficult for the turtle to get by without dropping the toy, or sinking under the waves.