Is growth an issue on your boat bottom? If it is, as I suspect, then growth in the heat exchanger is more likely than low refrigerant (which is also possible). Flow can look good even if the heat exchanger isn't capable of enough heat transfer.
Acid washing your A/C heat exchanger is a very reasonable DIY job if you are handy. It does require working with very hazardous chemicals and could result in serious personal injury if mishandled.
I'll tell you what I do and you can decide if it is appropriate for you.
I have a small (spare) submersible bilge pump with a long power cord ending in a cigarette lighter/power plug. Put the pump in the bottom of a large bucket, like a 5 gallon mud bucket. Disconnect the A/C cooling water outlet from the thru-hull, and the A/C cooling water inlet from output of the circulating pump. Using additional hose as needed connect the bilge pump up so that fluid flow will run in reverse. Put enough water in the bucket to dilute muriatic acid available at most hardware stores. I use "Acid Magic" which includes other ingredients that reduce the risk of damage to metal parts in the A/C. Remember: "do as you oughta, add acid to watah." Pouring water into the acid will result in spatter and out-gassing. Try not to breath the fumes - they are very bad for you. Run the acid through the A/C for about 15 or 20 minutes. I then use baking soda to neutralize the acid. If you don't neutralize the acid it qualifies as hazardous material and there are disposal issues. When you are done, run the A/C circulation pump manually for a couple of hours to ensure any residual acid is washed out.
Alternatives: remove the A/C (easy if it is a single-frame unit) and take it to someone who can acid wash it for you, or even better remove it and boil it (which will require recharging the refrigerant). You can also have someone come to the boat and do it all for you.
sail fast and eat well, dave
beware "cut and paste" sailors.
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