snorkeling, no.
i turn off both my strobes if im 15' or less, you cant really see them.
if you buy a strobe for later, invest in a good one.
i shoot TTL only, but some guys are fine in manual mode...i prefer not to have to fiddle with it, and its usually pretty accurate with Inon's. you can learn and practice on a single strobe, work you way up to doubles.
The G-15 is a very capable camera. get used to shooting in manual mode verses auto to get the most out of it.
if you use a LED torch, you'll end up with hot spots and back scatter in your pics.