I think it's more doable than it might look at first glance.
First I'd contact SUDS (Soldiers Undertaking Disabled Scuba) - they may have a list of charters that are service dog friendly if any exist. Or help you in other ways.
SUDS Diving | Soldiers Undertaking Disabled Scuba I would guess that some of these dive operators might be able to work something out also - probably have already:
SUDS Supporters | SUDS Diving | Soldiers Undertaking Disabled Scuba
Obviously most of Florida would be available to you. You probably can bring the dog on the plane subject to size restrictions right?
Conch Republic works with SUDS so might be a good first test - they're in Tavernier in the Keys.
Conch Republic Divers Fly into either MIA or FLL and drive down. Or there's both a shuttle service or the Greyhound bus. Miami is a cheap flight, non-stop 3hr. flight from St. Louis - under $350 R/T later this summer. Divers often stay at the Holiday Inn or the Marriott in Key Largo, there's also a Courtyard and a Hampton Inn nearby - about 1/2 dozen dive operators that leave from the dock there if you can leave the dog nearby.
You'd probably need to get your dog a life vest at a minimum and provide some means for someone on board to control it while you're gone - I have no idea what that would be. Crate? Another family member traveling with you? I'm pretty sure the Captain isn't going to want to be responsible for the dog while you're gone.
Another possibility might be
certain Divi
Dive Resorts. I know the Divi Flamingo works with disabled divers - but it's on Bonaire which may pose it's own set of challenges both with multiple long flights to get there and IDK their service dog importation policies.
You might check with the Divi Carina on St. Croix to see if they have similar arrangements - it's still the U.S. and afaik there's no import restrictions on bringing dogs there with the proper medical clearances.
St. Croix All Inclusive Beach Resort | USVI Vacation BEach Resort | Divi Carina Bay All Inclusive Beach Resort & Casino The problem with St. Croix is that from some origins, they use much smaller regional jets on connecting flights so I'm not sure how that would work for the dog.
Dive Resorts Service Dog Policy - I'm sure many resorts in good dive locations do also - maybe start with some of the SUDS sponsors in U.S. cities first. Or Puerto Rico, it seems SUDS recently had a trip there.
Many airlines use smaller regional jets once you get outside of major destinations. For that reason you may want to consider either St. Thomas (also very good beginner diving) or Puerto Rico instead. St. Thomas seems to have reasonable flight times from St. Louis with a stop in Atlanta or Charlotte.
Taino Divers on Puerto Rico is a SUDS supporter - IDK anything about it.
good luck,
A cane or wheelchair doesn't require someone to watch over it while you're diving. On some of the smaller boats only the captain stays on board and he helps with re-boarding divers etc. As well as moving the boat following bubbles if you're drift diving.
Do dogs get seasick?