One thing to think about is a GoPro. Great video, you can stuff it into your carry on bag for your weekend diving jaunts (I assume you rent all your scuba gear when you go) and at least to most folks, video is a much more interesting way to interact with UW creatures.
If you really don't want a strobe then you can pick up a used G12/Canon housing to play with for a few $00 but if I were in your shoes I would go with the GoPro.
Bill
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I don't know what kind of pictures you take topside but personally (shooting a 7D/Nauticam/strobes etc) I was not happy with the quality of the G12 UW (shutter lag is one issue) even shooting strobes. Without strobes using the on-board flash it was even more grim (to my mind), but you can do the experiment with used gear for a few hundred dollars). To me, having been in your situation is to avoid the inevitable creep of getting better and better gear. You will buy the G15 and Canon housing for $700. Then you will shoot for a bit and decide you need a strobe ($500 at least plus tray plus FO connector so $700 total). Now you are in for $900 and have the same strobe you could have for the 5D. For $600 (not insubstantial) more or less used you can get the Ike housing. It is true that you will need a bigger box to take on trips, the Canon and housing without strobes are quite small, but if picture quality is important then other systems make more sense. To me it is all about the quality of the pictures you get. That being said, if budget is the key then get a GoPro and shoot video and take some still captures from that.
Bill