A typical 2-tank Cozumel boat dive trip is roughly 3 1/2 hours. It's hardly all day. A one-tank dive (night dive, for example) usually eats up nearly 2 hours, so I really think the "normal" morning 2-tank trip is very much worth doing.
My son's girlfriend had her doubts but we twisted her arm. Now she has a full set of dive gear at the house and tends to push us to go diving when we want to roach around the pool. This is the true problem with going out to the reefs, in my opinion. It's habit-forming.