We have friends that are cruising Mexican waters each winter on their Passport 40, and they say the watermaker is the best thing they put on board.
Other friends in the same situation are getting by without one, so the water is available but I'm not sure about the cost or across-the-board availability.
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"