#1 Check and clean engine intake thru-hull/seacock.
#2 Check and clean hose from intake seacock to strainer and any elbows that may be in this path. Large (very large) zip ties work well for this.
#3 Remove strainer basket and snake both inlet and outlet ports.
#4 Remove hose from strainer to water pump and check/clean
#5 Remove and check or replace impeller and also check the old one for broken vanes. If vanes are broken you NEED to find the pieces.
#6 Test run engine make sure you have good water flow. If you are still over heating move on.
#7 Using an infrared thermometer (you can't effectively trouble shoot a cooling system without knowing actual temps!) measure the temperature at the sending unit, on the thermostat housing, and compare that reading with your temp gauge. Also, measure the temperature differential between the inlet and outlet sides of the HX and write down the differential. If the gauge and infrared are close and high remove and replace the thermostat. If you must, and are of the overly cheap mentality, remove and test the stat in a pot of water using the infrared to confirm opening temp and re-install.
#8 Make sure "sealed" side of the system is not air bound and use the air purge valve to check for any trapped air.
#9 If still overheating check the temperature of exhaust elbow (on the engine side) and about 6-8 inches from the exhaust elbow on outlet side for a large temp differential. If you have a large temp differential you have a blockage at the exhaust elbow. Fix this before moving on and re-check engine.
#10 If exhaust elbow checks out open up the heat exchanger and look for "gunk" if junk, corrosion, pieces or crap are present remove the HX and bring it to a radiator shop for an acid bath cleaning.
#9 Re-install HX and check engine.
#10 If still overheating check for flow and then if it seems restricted purge and clean the "sealed" anti-freeze system side. It's possible the journals and pathways are loaded with rust and corrosion and you have a partial blockage.
P.S. When the engine is finally fixed and running well use your new infrared thermometer to record the following numbers in a notebook. This will save LOTS of time when you have a cooling system issue in the future..
HX Temp differential between inlet and outlet sides.
T-Stat housing temp
Exhaust elbow - both sides
Exhaust hose temp
Various points of the cooling system
It's best to mark every one of your infrared thermometers "laser" measuring spots with a Sharpie marker so you can repeat these numbers and readings accurately by always measuring the same exact location.