It sounds like an exciting trip you're planning! Getting off a tender port can sometimes be a bit unpredictable, but aiming to be among the first in line to tender off the ship is a good strategy. Assuming everything goes smoothly, and depending on how quickly the tender process is managed by your cruise line, you could realistically be on shore by around 8:30am if you catch the first tender, or 9:30-10am if it takes a bit longer. So, targeting the 9am water taxi might be a bit optimistic, but the 10:30am should be very doable.
Given that there are two other cruise ships in port that day, the water taxis, especially the ones heading back, might get quite full. It's wise to consider the 3:30 pm taxi back to be on the safer side, but also be prepared for it to be crowded. If you prefer a less rushed and potentially less crowded experience, aiming for the 1:30 pm return could give you more peace of mind, though it does shorten your time on Caye Caulker.
With the 10:30am taxi to Caye Caulker and planning to return by the 1:30pm taxi, you're looking at around 2 hours on the island. This is a relatively short time to experience everything Caye Caulker has to offer, especially if you want to explore, eat, drink, and shop at a relaxed pace. Caye Caulker is known for its laid-back atmosphere, and it might be worth considering a longer stay if possible to fully enjoy the vibe of the island, visit The Split, and not feel rushed.
Considering there will be additional cruise ships and potentially more visitors to the island, planning your activities and being mindful of time will be key. If you're really keen on making the most of your visit and the tour with Coral Breeze fits into your schedule without the snorkeling, it could be a good way to see more of the area with the time you have. However, ensure it allows you enough flexibility to enjoy Caye Caulker's charm without feeling too constrained by time.
Ultimately, whether the time you have is enough depends on your priorities and how you prefer to experience new places. If a brief taste of Caye Caulker's beauty and culture fits your travel style, then your current plan could work well. Otherwise, you might consider spending more time on the island to soak in its unique atmosphere more fully. Have a fantastic trip to Belize, and enjoy every moment of your adventure!