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If you will be shooting video exclusively, get the GoPro. It spanks anything anywhere near its price point.... For stills.. not so much... its OK, but clearly not its forte. No RAW and its ultra-wide lens while being good on video, seems a little too wide for many still applications.... i.e. you won't be doing anything anywhere near macro.
The S95 is a fine camera for a beginner underwater photographer. Stills are great and video is decent. If stills are in your plans with some video, get this one. The s95 has full manual control (full aperture, shutter, and ISO control just like an SLR) and shoots RAW -- two very essential items in underwater photography.
Even if experienced at "topside" shooting, you will essentially be relearning photography..... Read a lot: http://www.uwphotographyguide.com/content/guide-underwater-photography