I'd suggest flying; you can make the first dive of the morning if you get in on the first flight, if you take the ferry you'll miss it. As far as photog goes -take as much as you can, but I'd guess you want middle to wide angle. Vis is pretty spectacularly clear, and the pinnacles are great. The twin otter also stays low enough you can safely fly back to St. Maarten after doing all 3 dives the day before - perhaps just notify your dive op and make sure they're aware and agree. As a matter of fact, try for the seat on the plane behind the pilots and have the camera ready. The landing is a bit sporty (very short, uphill runway).
Favorite single dive site was probably Diamond rock. Very narrow spiky pinnacle, maybe 80ish feet deep to the base at the deepest side (it sits in a bit of slope) and actually emerges from the water. Should be time to completely circumnavigate it at least once.
I haven't been for years, but search Scubaboard on my ID for my trip report on St. Maarten and Saba. A lot of it is kinda gushy as it was my first real diving trip, but Saba still stands up in my mind as superb diving; I'd put it slightly ahead of Dominica in my memory now (similar topography - very volcanic substrate with lots of coverage, but more larger schools at Saba than Dominica). We dove with "Saba Deep" which was a really small operation - two main guys (Big and Little Mike) and another DM who assisted . Most of the trip it was 4 divers on a little 6-pack boat, and they bring you back in between the 2 morning dives for a leg stretch and to pre-order your lunch, which was ready after the 2nd dive. With one exception (we were lucky enough to dive Green Island due to weather patterns up on the north side of Saba) almost all the dive sites are 15 min from the dock. They will do Nitrox but its probably not worth it unless you're waaay better at air management than I (you'll be air limited vs. Nitrogen at their typical dive depths, and of course for the first and deepest dive of the morning, it's too deep period).