So far the user joinable groups one can join that changes the default user title from "Registered Member" to something else are as follows.

Cave Diver
Instructor
Photographer
Videographer
Wreck Diver

Maybe we ought to add rebreather diver. We also could investigate a little more about paying for a custom script to be written so some of your user profile could be under your avatars, like # of dives or whatever....