I'd give it a detailed look over, checking the condition of the membrane cases, hoses, gauges, and motors. Watermakers aren't complicated but they do have some basic requirements for operation.
The three biggest concerns I'd have are the membranes, motor
, and high pressure pump.
You can get a good idea as to the condition of the membranes by doing a salinity test. The kit's a few dollars and should be on board to make sure the product sensor hasn't gone bad. I don't know how many membranes are in the RO, but if you're serious about the unit, you may have to change all of them to get good product water.
Then I'd look hard at the motor
. Make sure it's in good condition, shows no signs of water intrusion, and spins smoothly. I'd also try to find out if the motor's 120 or 240. Power requirements can make it tough to run the unit at the dock
The other area is the high pressure pump. This is the real workhorse and heart of the system. Salt water
is corrosive and high pressure water, even more so. If the manual shows you how, I'd consider taking a port apart and looking inside carefully. The pumps aren't hard to rebuid but if the case's corroded or cracked, finding a replacement may be more expensive than buying
Since it appears that the original company has been absorbed by another I'd do some looking to see if parts are available, the RO is supported by the new company, and that the cost of the coin flip is worth it.