Regrettably, I've had both minor problems, 1) clogged raw water seacock with marine growth for the raw water side, and 2) cracked overflow tank hose for the coolant recovery tank ( in the lazarette, because of the in line water heater on the coolant side) which combined to raise the operating temperature without any apparent leak until I tired to fill the overflow tank from above and noticed the coolant dripping off the lower loop of the tank hose.
The actual crack in the hose was beneath the tank at the nipple (which again, is in the lazarette above the coolant tank and above the water heater) and dripped only the overflow to the shaft log rather than the engne compartment or bilge.
Clean the seacock from within with a bore brush to ensure adequate raw water flow and then check the coolant recovery system at or near your water heater.
Hope this helps. By the way, this is on a Universal 25XP ( actually the Kubota 950 3cyl block).