I wouldn't worry about pre-booked, as august is off season and things can be cancelled easily and re-booked to fit a better schedule,
Pre paid is different, though . . .
Also, if you like nature things and this is not necessary a full-on dive-only trip, there is a canoe trip near Orlando that you should not miss. This is called Rock Springs Run, and the outfitter that rents the canoes and transports you is called King's Landing canoe rental. Rock springs run is about 8 miles long, spring fed, clear, and goes through some of the most beautiful forests and wetlands you can imagine. Sometimes, it is like floating on air, due to the clarity. Usually lots of birds including wading birds. This is only 1/2 hour north of Orlando and easily reached. Sometimes deer, allgators, bear can be seen as well. But, the glory is the forest, clear stream, beauty of the area. I would swap this for a day at a theme park. In fact, I go to Orlando regularly and have not been to any theme park in 35 years, but hit Rock Springs at least twice a year.
There are also notably beautiful canoe trips near Crystal River if you stay there--Weekie Wachee River and Chassahowitska River are the best. Each is an easy day trip. Do not do Crystal River or Homosassa River as they are highly developed and lack the pure natural beauty.