Looks like everyone's a DS fan, I recently purchased the Inon Z-240 (type 3 - newest model), and can say S-TTL works darn well with the little p+s panasonic TZ-3. I know your post is old, but I'll say a couple things about s-ttl and optical connections, unlike the DS-51, the optical sensor and inons connections wont fire if others are taking pictures in your area, I think the DS-51 has a sensor thats pretty far away from the flash and can be triggered pretty easily. Additionally, with the Inon they included some simple films that you place over the cameras internal strobe which blocks visible light. The bonus here is that the cameras flash doesn't light up the water column directly in front of the lense producing backscatter, while still allowing the strobe to fire.
Since you are new to scuba, perhaps something simple to start, like s-ttl, wouldn't be a bad idea.
It can also be manually adjusted -2ev and +5ev on the back of the strobe, but rarerly have I needed to do this, or if you want to play with apertures shutters, e-ttl, the strobe handles those too. I like the idea of future upgradeability. (not the cost mind you!)
Just my 2 cents, hope it helps!