Helmet diving , also SEA TREK, inlcudes a "astronaut" type of head gear which rest on the shoulders and covers the full head. Air provided from tubes from surface baot. Lets participates to walk the ocean floor.
I checked a few Belize excursion sites and there is no mention of this type of diving. I wonder if it is because the coral reefs are very delicate and snorkeling is less of an impact. Sea Trek is a lot of walking on the surface which may be harmful. I hear Belize has the best coral reefs so I assume this is the best location to explore underwater.