Not to question Scott (as if!) but people win contests with the camera more than the housing. As usual, it will come down to your budget and ultimately what you enjoy shooting. A lot of folks knock the Canon housing but if it let's you afford a strobe you will get better photos (potentially) than the most expensive housing and no strobe own would.
If you have the budget to invest (a bit over $1K) it makes sense to start with a mid-level housing like an Ike and a decent strobe but you sort of are "stuck" w/ Ike strobes for full connectivity options. Not knocking their strobes, just that you need to use their brand on their housings for true TTL connections (an over-rated function but that is pure opinion

). If you can afford it, get a RecSea housing and one or 2 reasonable powered strobes (S&S YS-110A, Inon S2000 etc.) but now you are closer to $2K.
If you get a strobe you won't need a filter (more "opinion" but it does have basis in fact, at least for distances under 5' w/a stobe, or if shooting RAW--which of course my opinion sez you should always do!). As for wet lenses, the G-12 is an excellent macro camera that works great w/ many close-up lens systems like Dyron, SubSee or Inon etc. Due to the nature of the zoom lens and housing port though the camera is not that well suited for wide-angle work. WA lenses will vignette some at the widest focal length setting. If you have to zoom towards telephoto by even a nudge or 2 (to avoid vignetting) it kinda defeats the use of a wide angle wet lens. Doesn't mean they won't help at all, just that you are limited compared to say a Canon S-95. Oh and of course you'll need a way to hang all this stuff on the housing, so factor in an arm & tray set that will let you grow your system.
Before you get much further though, welcome to SB! There is a great search feature available here for previous threads and as you could guess, these questions have been raised before on all sorts of UW setups. You can really benefit from other people's experience, which is one of the main attractions of SB!. The G-11 uses the same housing etc. so, since it's been around longer, you may get more feedback by including it in your search. Have fun! // ww