Reddit reviews Benji Takes a Dive at Marineland

A diving suit was created for the dog star Benji and Marineland is the location of the historic event of the first scuba diving dog.

There was a book published to document this event.

From IMDb:
Here now is one of Florida's very rare (if not lost) motion picture gems, and a ton of fun for the whole family. Filmed in 1981 as part of an ABC's "Afterschool Special" anthology series, this half-hour puppy flick served as a publicity stunt both for the Benji film company and for Marineland of Florida. Located on Highway A1A (the Route 66 of Florida's east coast) this Sunshine state counterpart to Marineland of California was one of Florida's first theme parks before Disney.

Besides Benji, the park has also shared the spotlight with Clint Eastwood in "Revenge of the Creature" (sequel to "Creature from the Black Lagoon")Lloyd Bridges in his "Sea Hunt" series, and Elvis Presley in "Live a Little, Love a Little." Alas today, the ravages of time, weather, and economics have turned the facility from a once famous dolphin show site, into a strictly marine biology education camp. Still, should you happen to spot this classic on DVD at your local Wal-Mart or Target Store, (along with the aforementioned other 3) snatch it up quickly before it's gone! You won't be sorry.