Slippery Fish
My all time favourite Sea Song is Slippery Fish by Charlotte Diamond. I use this song and we pretend to be each animal. The children love this song and sing it continuously throughout the day.

Sea Animal Limbo
I also saw the idea of a Sea Animal Limbo on another website. I played Slippery Fish or Baby Beluga and the children had to go under the broom stick like the sea animal.

River Jumping.
I place two ropes parralel to each other. The children had to jump the ropes. I kept moving the ropes further apart.