Sorry for a very late entry into this thread.
One idea I've been playing with is to use a remote flash trigger. Something kind of like a lightning trigger.
For example this:
http://www.doc-diy.net/photo/smatrig21/
...is a DIY project for a little device that plugs into the camera's remote shutter port. It includes modes for triggering based on sound, lighting (any flash), or even a laser-break with an included light detector. If you can fit it in the housing (it's pretty small, depending on the housing you might be able to), then swim a bit away, trigger by using a small manual flash thru the polycarbonate sidewall. Then you don't need any penetration of any kind.
Conceivably the sound-trigger mode could also be used, but since sound carries SO well thru water I'd worry it would be randomly triggering all the darn time. Plus any sound might scare whatever you're trying to photograph.
I would be worried about how quick this device goes into standby once closed in the housing though. Haven't experimented with it yet although I do have one. There are other lighting sensors on the market too, but most I've seen are both costlier and bulkier.
Of course if your actual strobes are also optically triggering you may have an issue, as you dont' want to trigger your strobes with the shutter-triggering flash, you want the actual onboard camera flash (or the 'master' strobe) to trigger any slaves.
The laser-break option might also work but would be a lot tougher. Would require setting up the camera (on tripod on bottom or whatever), setting up a laser as well to shine on the sensor, engaging the mode, then interrupting the beam.