My opinion is that you get something much better than a Gopro for your budget. My wife uses the Gopro3, and it's just a toy without the fun you would expect from a toy: sudden defects (no flooding), short battery run time - when you use that miserable display you'll have only 30 minutes, watching your footage on that display is just painful and takes ages for loading, not to mention the filming limitations, you can't zoom, you can't stabilize, you can't get good low-light results....a long list. You already use it, so you know all that, but comparing to a decent video system you really get aware of the Gopro's shortcomings.
I've been using a Sony CX-760 camcorder in a Aquatica HD wave housing with a Keldan Luna 4 lamp for 3 years. Not a perfect system, but pretty good. All combined I paid around 4000 Euros, close to your budget. I'm sure there are some other good video systems in that category for your budget.