As others have indicated, it may well be related to cooling water flow. As someone who has found the correct answer often by exhausting all the incorrect ones, I agree and suggest starting with the simple stuff - the stuff that I failed to check initially when I had this problem and turned out to be simple to fix.
Find the cooling water inlet seacock and confirm that it is open (duh).
Check for a cooling water outlet seacock and make sure it is also open (ditto).
Look for water coming out of that through
hull (it's usually just above the water line) when you start the AC up - if you don't see a good stream of water then, yes, you have a cooling problem due to lack of water flow.
The inlet side will probably have a strainer on it near the seacock - figure out how to open the strainer (maybe close the seacock first!) and clean it out. I have found that strainer usually has some
grass and debris in it but have also found it completely backed up with sludge and other debris, especially after operating the AC in a marina
environment.
If you have a clean inlet strainer and still don't get enough flow, find the cooling
pump (usually the next thing upstream from the inlet strainer) and see if it's running when you start the AC - it should be obvious. Most of those pumps are very reliable with magnetic impellers and failures are pretty rare but if it's not running, you're not providing cooling to the
heat exchanger and it's going to give you that dreaded "HP" message.
If it seems like it's running but you're still not getting a good flow of water, the last test you can try is disconnecting the hose from the output side of that
pump and confirming a robust flow of water. If the inlet strainer was blocked with sludge, sometimes it can migrate through the strainer and foul the impeller on the pump. the flow coming from the pump should be quite strong; if it isn't you can try opening it up and
cleaning it out.
If you have open seacocks, a clean inlet strainer and a functioning pump and still are not getting water flow your problem is most likely upstream of the pump - a blockage in the circuit and there are no easy fixes for that. You will need to get to the water input and output of the
heat exchanger on the cruisair unit and debug from there. FWIW, I have had the "HP" message many times on different units and have never had to go that far to find and fix the problem.
Good hunting!
- Art