I believe your tour is scheduled for 8am Belize time, right? That would be 10am ship time (assuming that we haven't gone "fall back" because then it would 9am).
In the morning, the staff will let you know when you have arrived in port and will direct you to the area where they are handing out the "tender tickets" which are really just stickers. Most likely, this will be the Follies lounge. Because your tour is NOT through the ship, you will have to get your sticker from the person handing out "non-tour" stickers. Get to the Follies early enough, and you'll still be one of the first tenders out because they send people out based on excursion times, then add in the non-tour people to fill the boat.
Once you are in port, you will go left along the pier. In front of the Wet Lizard bar, there will be a little stand that says "Hammerhead Tours," which is also for Coral Breeze Tours. You should have plenty of time to do a little shopping before you need to check in. When you do check in, they'll ask for your lunch choice (if you chose to have lunch...and I HIGHLY recommend the chicken burrito...it was AWESOME) and you'll wait for everyone else to arrive.
Assuming that you don't completely blow off your check-in time, the excursion will wait for you a reasonable amount of time. As Heny says, tourists are the lifeblood of Belize...they very much want to make certain that no one misses their tour, that no one misses out on an experience because of a physical disability or fear, and that no one misses debarkation. Trust me, they know what they're doing and you will have absolutely no trouble.
A few more important words: make sure you wear plenty of sunscreen, especially while you're riding the boat to Caye Caulker. I was burned so badly that the skin on my face peeled in thick layers. I think that sweat, the water, the wind, the lack of shade, and the abrasive action of the sea salt erased my sunblock. Heny will tell you that sunblock on your face will cause your mask to leak...but don't listen unless you're absolutely certain that you don't mind feeling like leather! Also, I recommend taking your own snorkel and mask (I bought Body Glove, and it was terrific!) because you can ensure a good fit...one of the people on our tour had trouble with his mask (a novice and didn't quite know how to fix it when it came apart while in the middle of the sea).
Okay, I hope this answers your (and anyone else's) questions. Have a great time on your trip and tell Caye Caulker I said I can't wait to visit again!