If you can't find a guy with that specific experience and have some mechanical aptitude yourself, do some web research like here-
Cruising watermakers for sailboats & power boats | Sea Clear Watermakers, here
https://www.spectrawatermakers.com/us/us, here
Village Marine Watermakers - The World's most trusted RO watermaker manufacturer, and especially here-
Yacht & Boat Water Maker by Cruise RO Water & Power
This will give you a good introduction to watermakers, so you can make your own mind up about power, capacity, integration, automatic features, etc. Then the installation is just plumbing and wiring like anything on a boat.
Even if you do find a specialist, he will probably have his own prejudices and left to his own devices you may not get what is right for you. So become knowledgeable so you can have a better conversation with him about what fits your needs.
And finally, where will you be in the Bahamas. It has been quite a while but we spent 10 weeks in the Abacos about 15 years ago. Every 2-3 weeks we would take a slip in Marsh Harbor to do laundry, reprovision and fill the water tanks. Water was free at marinas in Marsh Harbor. Even if we didn't stay in a slip we would fuel up and water was then also free.
And finally, finally water won't be that expensive for 8 weeks in the Bahamas even if you have to pay for it. A couple, if frugal will use 5-10 gallons each day. That will cost no more than a few hundred dollars each month, a fraction of the $5-10,000 a water maker will cost to install.
Edit: Just checked Active Captain and water is available at a couple of Marsh Harbor Marinas for $5.00/day. Other areas- Green Turtle Cay, Abaco- 25 cents per gallon and Georgetown, Exumas- 40 cents per gallon.
David