Need help with temperature issue

Thanks for the replies. I will try the above hose method and check all my hoses and report back.

180* isn't hot ? The T-stat should open there and the temp should drop - and repeat.

Since it takes a while to start getting hot I suspect you have a hose that is leaking air into your water intake. It is sealed when cold but as the hose softens due to a little engine heat it starts sucking air instead of water. As a test, take your garden hose and put it in the raw water feed hose, Turn the water up high for this test, run the engine and see if the temp ever rises. If the temp stays at your thermostat opening temp of 155 you found your issue.
If the temp still climbs you need to look harder. The garden hose spraying into the inlet hose rather than depending on vacuum will eliminate the possible vacuum leak possibility.
If the hose solves the heat issue you will need to search from the inlet at the boat bottom and follow every hose from that point to the raw water pump.
It may go to a water filter and to the tranny oil cooler prior to the raw water pump, check each hose and each clamp, if you torque the clamps while the engine is warm you will get a better seal. Warm soft hoses clamp and seal better than cold hard hoses.
If you have a water filter the gasket or Oring on that filter need to be checked to make sure it is sealing.