I think the basic answer is that you don't usually need a strobe for snorkeling photography (well, at least at the start).
If you are photographing at depths that many of us snorkellers do ie rarely deeper than 5 metres, there is usually tons of ambient light so no flash is necessary.
Even when fish or critters are under ledges or coral overhangs there is still quite a bit of ambient light so the camera's flash is fill-in more than the dominant light source.
Now, snorkelling at night is great fun and can produce some neat pics. In that situ I suspect a strobe would really help. Having said that, I've found that relying on my buddy's use of their torch produces some lovely effects without a strobe.
Hope this helps.
Cheers - Errol