I like the Sea & Sea YS strobe range a lot. I've recently bought the YS-27 from ebay (seller's called Nyxell, very reliable guy). It works optically via an optical cable which you put on the outside of your housing opposite the internal flash. I connected mine by pasting some velcro on the housing and sewing the free end of the optical cable onto the other side of velcro.
As for tray, any tray with a standard tripod mount will do. Just make sure you can connect the arms to the tray.
I haven't done a dive with the YS-27 yet but I have used a YS-110a I borrowed and our office Canon A630. Here's a shot with the setup

Exposure: 0.008 sec (1/125)
Aperture: f/5.6
Just the standard sea and sea tray and arm setup.
The difference between the YS-110a and the YS-27 is that the 27 doesn't have an internal focus light and only functions as a slave. It's also a bit less powerful and differs in other technical ways I'm not sure about. I plan to get a YS-110a after I buy myself a tray and some arms to compliment the YS-27.