When we purchased Magic 7-years ago the forward air conditioner raw water output appeared slower than the other two units (separate outputs, one raw water pump). The surveyor did not call this to our attention and it wasn't noticed for awhile. I thought it was normal for the most distant from the pump of 3 overboard discharges to run slower than the other two. Over time it got slower and slower.
An A/C repair company was called in to fix the issue as the output became just a dribble of water and we needed to shut down the forward (master stateroom) A/C.
The first repair attempt was to blow out the hose to the unit with compressed nitrogen. This fixed the problem---temporarily. Within about a week the problem reappeared.
The next attempt was to circulate Barnacle Buster through the system. Again, it worked---temporarily.
Next we replaced all the raw water A/C hoses from the sea strainer to all the units. Hooray, that worked---temporarily.
I theorized that the seawater pump which was 23 years old should be replaced. The line was blown out again and the pump replaced with an Oberdorfer 1/3 hp pump. Well, guess what, that worked--temporarily.
Spoiler Alert; If you want to try to figure out the ultimate solution on your own, read no further. If you want the answer just scroll down a bit.
With the A/C company blowing out the lines yet again, I decided to crawl under the cabinet in the forward head to work the ball valve for the discharge line back and forth a few times in case there were barnacles or other debris caught in it. I rotated the handle a few times but felt no resistance. We removed the ball valve and discovered that the handle had broken free of the ball in the valve leaving the valve only about 1/3 opened. And, the opening was clogged with debris that couldn't fit thru the restricted opening. This explained everything. The valve was replaced and for the first time we have full water flow. I also have all new hoses, a new pump and a bunch of receipts. And so it goes!