Probably it's the T-stat
It's not clear to me from your post if you have good water flow from the exhaust or not. If you do (the flow looks normal) then the two most likely causes are a stuck thermostat or a clogged heat exchanger (coolant side). It's probably easiest to just replace the thermostat first. They will often open a little, but then get stuck. When you are not running the engine hard it will work just fine, but when you are at full cruising speed it will overheat. Sometimes they will work loose after a bit, but you should probably just replace it. If that doesn't do it, you may need to pull the heat exchanger ore and have it dipped at a radiator shop.
If you don't have good water flow and the strainer and impeller are okay (look at it closely), you could have a clogged heat exchanger core (saltwater side) or a partially blocked mixing elbow.
My money is on the thermostat. Good luck.