Looks like the Fantasea option is well worth considering for you,the biggest plus for the FG16 is the access to the front dial on the camera which you will definitely appreciate if you plan to shoot in manual (this dial provides shutter speed control) or TV, Av, P mode with ease. If you're set on the g16 dont go for a housing that sacrifices any of the controls (dont go for the OEM, essentially). As I see it, the strength of the canon G cameras is having a superior user interface, ie. more dedicated buttons/dials at your fingertips, than many compact cameras offer. This combo would serve you very well for macro and fish portrait shots, particularly with strobe(s) and macro lens.
The big difference performance-wise in going with the Fantasea over the Recsea or Nauticam (both of which are 2-3x jump in $$$) is that the Fantasea uses a separately sold (albeit not too expensive) 67mm adapter which clips on to the square port to allow wet lenses like a macro diopter to be used. Its an odd design that works well - but its not as simple and necessarily as sturdy as mounting lenses on the threaded ports of the aluminium housings.
I have the FG15 for my g15 and one other big thing to consider with these cameras is whether you want the ability to do proper wide angle (100 degrees + field of view). The inon s100 zm80 is just about the only wet wide-angle lens for the job due to the particularities of the G cameras' long focal range. In all my searching around to date I havent been able to find evidence of someone using the zm80 with the Fantasea housing, and some people seemed to suggest that the adapter is not suitable for attaching a relatively heavy lens such as this. Which means for any wideangle shooting you might want to do you will be limited to Fantasea's own bulky Big Eye Dome which only restores the 75 or so degrees angle of view it is on land (to the tune of a couple hundred bucks).
However, i tested out a friends zm80 on my fantasea housing with the adapter and decided it felt and looked good enough to take the plunge in buying my own. The 67mm eyedapter has a surprisingly fast hold once clicked on. I will be diving with it this weekend and hope for success, and will put a post up about how that combo works for the benefit of others, since there seems to be nobody else whose explicitly described pairing the Inon zm80 lens and Fantasea housing (DO tell if you have, anyone, or know someone who has!!)
If wideangle is not a priority then none of this will be a problem for you, but just thought you should know if you end up settling on the Fantasea Fg16. Also I use 2 sea nad sea YS-01 strobes with my g15 and the are terrific, powerful small strobes that are very user friendly. They can be found for about 300usd on Divervision I believe. YS-01 does sTTL, or for a cheaper option that requires you to set the power manually full time the YS-02 is essentially the same thing.
No matter what you decide to get......just learn how to use it. Lots of practise makes for better photos.
