We sailed on the Sea in Feb/04. My advice:
Cozumel- don't miss diving in Coz- Your mom in law will enjoy the sites and friendliness of the locals there- you should have enough time to hopefully hang out in Coz together after your dive.
Roatan- You dock in Roatan, but she will not want to wander around there without you, it could be overwhelming. You should really dive in Roatan, catch a cab to the West End (we hooked up with about 7-8 other divers from our ship, we met them that morning while waiting to get off the ship- the other divers are pretty easy to spot, and they will almost always want to welcome fellow cruisers to go dive with them if there's room). Turned out that we were all going to the same dive op anyway! (Coconut Tree Divers in the west end). The boat ride to the dive sites is 10 mins or less. very nice. And they were willing to take afternoon divers and get us back in time to catch our cruise ship, I had emailed them prior.
Belize- we didnt dive there, were going to save our $$ for Cancun diving. Well, this was a mistake- when we woke up on the Cancun day (last port), all bright eyed and bushy tailed, dragging our big bags to the debarkation site on the ship, we were told that because of the high seas, we weren't stopping in Cancun at all. I've read about another person that this happened to also, so don't count on diving there. Get it out of your system at the other sites, then if you get the good fortune to stop in Cancun (tender boats only) you may get to dive, yet. There is good diving there, btw.
Let us know how it goes! Have a blast, this is a good itinerary.
Foo