The ways to hook up a flash are optical or wired.
Do you want to use TTL (automatic strobe exposure control by the camera)? Or are you comfortable doing manual exposure control?
It also depends on whether you want to do mostly macro and fish portraits, or trying to do wide angle scenics. For wide angle, you'll need 2 strobes. Otherwise you can start with 1.
Probably you should start with 1 strobe, and TTL.
If you want to stick with your housing, wired is out. If you want to go wired, then the ikelite housing and an ike strobe, such as a ds51 to start, would be your main option.
Ikelite stuff is relatively inexpensive, and good quality. They also have fabulous service if you ever do have any issues.
So for optically triggered, the rest of this message applies.
For Under $400 (street price) including tray and arm, the af35 is probably the best cheap way to go, though it is not very upgradeable. But you can buy 2 of them, one on each side, which would cover basic wide angle with the stock housing and lens.
If you want a more advanced optically triggered strobe that uses a fiber optic to trigger the strobe, and a tray and arm, it will cost more. The most popular choice would be the inon s2000, which is around $470 plus tray and arm, ($120-$150). This is a great strobe, very light and tiny, and good TTL optical slave capabilities.
After inon, sea & sea makes some smallish strobes that also offer TTL via fiber optic, though they do not seem as compatible as the inons...
Another choice would be the ikelite ds-51, and the heinrichs optical slave trigger, which together would give you TTL exposure control. The strobe is $380, and the heinrichs is about $150 or so.
Here is a complete list of all the uw strobes and their cabibilities:
http://www.digitaldiver.net/strobes.php
Click on 'preflash compatible' and see what you find.
The guys at Reef photo & video help to support this site, and they handle all the main contenders I've mentioned here, and can set you up, and are great to do business with.
Happy holidays.
Don