It is very easy to add a reasonably reliable thermocouple-type temp
guage to almost anything - and even quite cheap if it is not "marine."
Once could even easily have multiple switchable thermocouples to
monitor potential trouble spots, like that heat less-than-ideally
arranged exchanger, if desired. Raw waterflow is only one potential
overheating/disaster cause, & by your descr you have no basic
instrument to help protect. Hand-feeling the engine once per watch (or
per day trip, your call) helps verify that the temp guage is
working/reading within range. Larry can probably tell you how to make
a good one for $25.