I'm not trying to be smart but you can probably get much better answers on a scuba or underwater photography forum not a speargun mfg's forum.  Try scubaboard.com

Sinced you asked though, there are some good sales out there for very basic cameras.  On sale for around 110 bucks, is a 6 mp/camera and case that works down to 180'.  http://www.divers-supply.com/I…..-P926.aspx

Not anywhere near the best but certainly capable of seving its purpose for recreational picture taking.  And if you flood it, not that big a deal!  You will pay 2 to 5 times that amount just for a housing from Ikelite or similar mfg's. 

A buddy spent 99 bucks for a panasonic/lumix 8mp camera (which by the way has an underwater picture setting and video) and he found the case for 89 bucks on sale, online.  So for right around 200 bucks with shipping he has a very nice setup that takes pretty good pics with ambient light.  If you really get into it you will need a strobe.   Look around,do some searches and you'll find something decent without breaking the bank.  Keep an eye on ebay; every so often great deals show up there.

And to keep this relavent to this website, here is a camera mount for your speargun:

http://www.bluetunaspearfishin…..Mount.html

Disclaimer:  I have no interest in any of the businesses or products mentioned but like you had a hard time finding a decent camera under 800 bucks.

Good Luck!