My horsey niece was playing in the pool, balancing on the rolled up cover and an inkling was born.
How about a inflatable seahorse with internal plastic pieces that lock on inflation? The pieces form a solid skeleton to allow weight to be vectored around the inside of the toy.
Using weights, that can be placed in the head, tail and inside the body, gravity can played against buoyancy for a very unstable and exciting ride. Different weights, positions allow for varying levels of difficultly. The name of the game, for Yee-horse, is stay aboard. Last one off is the winner.
sploosh, 1m 34 s - winner!-- wjt, Dec 02 2015random, halfbakery