There is no danger in booking far in advance as you can cancel if you need to. It also gives you priority in case they have other bookings. Peat Taylor is great! I highly recommend his company. For Sting Ray City, I've used Native Ways and Sotos Cruises. I prefer Sotos, but both are fine (Sotos gives you 3 stops vs two).
BTW: you are doing the right thing by planning in advance. Sometimes on the islands you can get by with "seat of the pants" planning, but you may also miss a lot of things. You have such a limited time on the islands, it is at least advisable to know what the key activities are. Once you know know what is available, then you can book in advance or find a tour operator once you get there. In my opinion, it's better to book ahead. Many times you can research the operator to get an idea of the kind of service/tour they provide. This web site is excellent for that!