One of my favorite shots to do at the beach is too have models running and splashing in the ocean. My favorite is when the they run right at me, I put my 1DsIII's AF in servo and with 85 @ 1.2 and click away. It locks on and stays on the model. It looks very cool to see all the water splashes with varying degrees of OOF and then the ones that are in focus look very cool.
Not always the fastest moving subjects but it gives me a lot of sharp sharp keepers.