You're right, TTL will not work with that setup. That's OK... UW, manual is better. Most of your shooting will be at 1/60-->1/200 and F5.6 --> F11. I like to set my f-stop for the DoF I want and then vary strobe power to control exposure of the foreground and shutter speed for the background as subject motion permits. Lighting underwater is vastly different than topside. Strobes are only good to a few feet -- even powerful ones, and depth eliminates warmer colors. By about 100' its all blue so if you want white light, all you got is what you brought -- your strobes. I suggest a lot of reading in this area. The book "Underwater Photographer" by Martin Edge is an invaluable resource. He does an excellent job of covering lighting and many other techniques.
The YS-D1 is IMHO, the best strobe out there right now for any reasonable price. Seacam strobes are better, but they're also $2500 a piece. Get the pair because eventually you will want/need them. Learn how to shoot with one first, however. Like in the studio, you should know how each light affects the image before you add more. See above comment about UW lighting being vastly different. When I started, I thought that I knew what I was doing since I regularly use 2, 3, and even 4 strobe setups in the studio mixed with window light, etc... After my first few UW photo dives, I realized I was learning light all over again.
I don't (seriously) shoot video, so I can't help much there. Unless you plan on diving very shallow, you will need video lighting as well for the same reason mentioned above. Video below 30-ish feet takes on a blue cast that's hard to impossible to correct with white balance -- the warmer colors just aren't there, so even if you tone down the blue cast, other colors will look gray/muted.
IMHO, Ultralight Control Systems (ULCS) makes the best arms, clamps, balls, etc.... I suggest an 8" plus a 5" arm section plus clamps for each side/strobe. This will give you all the articulation you will need and allow you to place them far enough away from the lens axis to control backscatter.
Lens-wise, you may want to consider a 100mm macro and flat port. There are a lot of macro and small subjects that you will miss with the lens you have. Also a zoom like the 16-35 or 17-40 will be more versatile than your 15 prime. The 15 will be good for video, close focus wide angle work, and larger animals such as turtles, sharks, sea lions, etc.... I've never been too fond of the fisheye look (underwater or above), but that's purely personal taste. I also shoot a 5D2 and my UW lenses are the 16-35 2.8L II, 24-105 F4L (mostly used in the 24-70ish range), and 100 2.8 Macro. The zooms go behind a dome and the macro behind a flat port.
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