Try resetting the sim with resetsim *
Did you send him snorkeling a bit first ?
Here's some info although yours does sound like a bug: http://www.carls-sims-3-guide.com/skills/islandparadise/scubadiving/
Are you using mods in your game ?
I have the same problem, just started happening (worked before). No mods ever. ResetSim didn't work.
I'm thinking it has something to do with Into The Future. Been some godawful glitches since that one was installed.
Is it just one Sim or all of them ? Did you try moving him out and back in ?
