It depends (don't you hate that answer) of course on your goals, current setup, and where you think you might go with uw photography in the future.
As giffenk points out the inon strobes are highly recommended as well so you'll have to decide if you prefer details about the inons or the sea&sea with your particular camera.
I recently added a second strobe and was deciding between the ys-01 and the ys-d2. The ys-d2 is more powerful but is also about $200 more expensive. I ended up choosing the ys-d2 over the ys-01 because with my specific camera, the ys-d2 works more seamlessly in both ttl and manual whereas the ys-01 requires much more configuring on my tg-4 to get it to sync well with the f/o cables. So, make sure you verify the compatibility of your camera with the strobes.
If you have a WAL or expect you'll shoot a lot of wide angle more power (and two strobes) will be useful. If you expect to mostly shoot macro/close up than a single strobe and ys-01 might be more than sufficient.