I think I'm going to go with this
http://www.circuitcity.com/ssm/Sony-...oductDetail.do
And build a DIY housing out of PVC.
I see a couple potential problems with the camera.
One is the location of the zoom/wide angle slider. It's on top and sort of recessed so I think it will be harder to move with a diy control should you attempt to make one. And the rec start/stop is on the back so it will need to be pushed from a control mounted thru the back glass, not the tube of the housing. These two controls would be the bare minimum I'd like to have u/w, you can let everything else work auto(focus, white balance. etc) and still get good results in bright clear water.
Also the viewfinder seems rather small, so unless you're going to diy a housing that has the lcd screen open, it will probably be necessary to position it as close as possible to the rear glass. Since the battery mounts underneath it, using any sort of extended life battery will move the viewfinder forward a corresponding amount.
Also Sony deleted the Lanc control jack on this model(strange, it was on the lower end HC21 last year) so there's no way to use this camcorder if you decide to upgrade to an electronic housing later.
One good feature is that they put redundant controls on the LCD screen. I could see engineering a housing with an Ikelite pushrod that could hit these controls with the LCD screen open similar to what Ikelite does with their HC1 housing.
Might want to read this review also:
http://www.camcorderinfo.com/content/Sony-DCR-HC26-Camcorder-Review.htm
Is there a better camera for around $300?
Probably not, the Canon rates worse. I don't know about the Panasonic. Although either of them will also have the lack of LANC control problem.