Found the culprit. My Force 10 water heater has a Watts temp regulator attached to the hot water output. Removed at the output side and a small screen fell out along with what seemed to be boulder size gravel. Cleaned out the fitting and the screen and reassembled. Everything works perfectly now!
Driller B, thanks for the recent additional info.
Unfortunately, the clog wasn't in the screens in my Watts temperature regulator, but in the Kuuma water heater itself: only 14 months old, and it is already clogged with scale.
Not surprisingly, our dealer says Kuuma's warranty doesn't cover this. Kuuma's engineers suggested using vinegar to try and clear the tank, like you'd do with a coffee maker. Apparently, nothing stronger than vinegar is permitted.
We are all puzzled about the scaling. None of the other boats at our marina have had this problem, including several newer Hunters in the SailTime fleet, and we've only filled our water tank with water from the marina where our 2009 Hunter 38 was commissioned, and our home port marina.
We are planning on replacing the Kuuma with an Isotemp Slimline 20, and hoping we don't have a repeat of the scaling problem.