What the others said.
I would differentiate between boats that concentrate on the diving and those that cruise with some diving thrown in. With the concentrate on diving boats, you have a greater than average chance to have a more experienced group with more interest just on the diving. Definite 4 dives a day, sometimes with a night dive too people. This has its advantages, but it's not an ironclad guarantee. This type of boat isn't for everyone. Personally, it's the only way to go. I concentrate on diving and nearly get my fill (never enough) then I can go off on the rest of the trips activities. Then if anything screws up the diving, you can always bump the other activities to get in the diving! Not for everyone.
There are differences between the boats as to how the handle the dive schedule. Some take the group out and dive at Time X. Miss the boat and you miss the dive. Others open the pool and you dive when you want. Others have basic dive times but you join when you want.
That said, the time on the boat is your own.
You also have more choice than the list of choices offered above. Decide where you want to go and check out what's available.