I have never properly scuba dived with a tank, but I've snorkeled plenty of times. I'm torn between two favorites.. One was between the Galapagos Islands. The water was really cold, but the scenery was spectacular. The other memorable time was snorkeling in Akumal at a time of the year when the green sea turtles were grazing on the seagrass. It was really cool to swim next to them.
I've considered gotten into scuba diving so that I can explore the cenotes in Yucatan. But when I looked into it I saw that cave diving requires quite a bit of training, and it takes some serious effort to get there...