Hi Gilarax, I have limited experience with underwater photography, but I guess the answer depends on how deep you're intending to go and how far the subject is likely to be. If you're at snorkel depth on a bright day and the subject is close, then you'd probably be fine with the built-in flash, or in some situations, no flash at all. But any deeper, dimmer or further away and you'll need additional power.
While searching for housings a while back, I noticed a number of specialist dive sites which would probably be better for this kind of thing - I'd search for 'G9 underwater' and take it from there. Do come back and let us know what you find though, and please do share some of your images too!