A few romantic ideas :
Have a private meal served to you
Hire a taxi for a night drive around the island, Stopping at a few places to get out and enjoy the view and each others company.. You and your husband , no need to include the cab driver..he or she will understand ...LOL
If you are diving on his special day, compose a special poem on a dive slate and present it to him at a special point of the dive. Or, even a paper copy will do, perhaps put it in a plastic bag in order to protect it for the trip underwater.
Since signing is the main method of communication for divers, maybe learn how to sign "Happy Birthday"
Take along or arrange in advance to have special treats for your surface interval.
head down to the Money Bar for a Saturday Night Birthday Celebration
Whatever you decide upon, just being together and sharing the day is wonderful. Best Wishes to you both. !!!