I have a sunpak g-flash, which was only $200.00. It has modes for digital preflash or normal camera mode, as well as a dialable power output. It wasn’t very easy to find, but it could possibly replace your existing out-of-water flash. It also came with the frame to attach to the camera housing and also came with a diffuser. That may save some money rather than buying both a flash and flash sensor. I use this with the coolpix 4300 and the fantasea housing.
I did a ton of research before buying this setup, and for the price, I feel I got the best value available. If you are in the market and confused, I hope that helps.
The cable solution for digital cameras, for those of you wondering, is a piece of fiber optic cable that attaches right in front of your existing flash. It then attaches to the flash sensor on the external flash. However, it has a device that waits for the two flashes from the camera before flashing 1 light at your existing flash to trigger it.
Hope that helps,
Matt