If you have an ADR at the Coral Reef. You can call the rest. and ask if they have any time slots open for the diver. I believe they do 4 or so a day.
A diver will swim in the tank and hold up a sign saying whatever you ask for... Happy Birthday, Anniversary, Will you Marry Me? And then they will give you the sign afterwords. They don't charge for it. But snagging one well good luck w/ that!