After looking up and down the board, you have lots of questions on planning these excursions, Imgoingtodisney!
Besides the literature, Here's a website on Coral World:
Click ME!
Now, it's pretty far from the ship, and I really don't think you should try to squeeze in CW and time on a beach and time shopping and the sub. I agree with Dave, with previously seeing pics from the sub, you're not missing out much. We used up our day between CW and the island tour and shopping and doing the tramway, which the shopping and tramway are right at the port. Now, we had an unusual situation, as there was some kind of big funeral that was tying up half the roads between CW and the port, so it took us longer. But still, don't rush too much, you'll miss out on some of the fun that way.
Now, about the casinoing on St Maarten, we did the See & Sea tour right off the ship when it docked, and we had LOTS of time to kill even after shopping and doing some casino visiting, so I wouldn't worry too much there, you'll have time for what you're suggesting without any rushing around.