Help with sudden RWC overheating diagnosis..

[TimBSmith]

Sailing out of Salem, MA this afternoon / evening. Monitoring temperature and and exhaust as I always do. Flawless operations for weeks after my fuel system pressure issue.

-No thermostat. Diverter valve. Usual run closed.
-Raw water cooled.
-Rarely breaks out of 170/180.
-Water batches reliably, basket filter bubbles briskly.

This afternoon we found some nice wind and decided to turn off the motor.
When I went to check temperature (I had last checked maybe 30 minutes earlier). Temp had risen to 208+ F across all spots I check on head.

-I went into trouble shooting mode.
-Check basket, clear.
-Checked impeller, wet, intact and hot water leaking from backing plate when I removed to check impeller. (new impeller last year).
-Marine exhaust hose from water lift hot to the touch and maybe felt firmer than usual. (New last year).
-All hoses forward of the water pump were hot to the touch.
-Confirmed flow from inlet through hull to basket filter.
-Oil level full and no sign of water.
-Transmission case felt hot.

-Restarted engine, no choke needed, opened intake through hull, no noticeable water flow or bubbling through basket filter. No water leaving the external exhaust.
-Shut motor down.
-Closed feed.

Wind picked up, a blessing, we sailed back to mooring in 12 Knots, steady south westerly. Oasis sailed like a dream, thought we might need tow service, but NOT ON THIS DAY BABY!

Back on mooring. Tidied up regular post-sail procedure.
-Started engine one last time as test.
-Started first crank with choke (You go girl!)
-Opened feed.
-No joy on water pressure to basket filter.
-Did notice leakage from pump backing plate. (I may have reinstalled the thumb screws too loose or didn't seat the gasket flush (maybe screw cross thread.) Still love the thumb screws!

Tomorrow morning going back to the boat to resume diagnostics.

Question:
-I will search list for signs of a failing or failed water pump.
-What diagnostics would you offer as I check the cooling system end-to-end tomorrow? I don't have exhaust back pressure gauge.
-How do our pumps usually fail. Do they seize-up or do they simply slow down and create less pressure? What are symptoms of a failing or failed water pump? I vaguely remember reading about a weep hole, and oil leaks from accessory drive? Investigating. Welcome thoughts.

Thank you brain trust. Until next. Stay well.