The cam plates do wear down on the seawater pumps and I've rebuilt them many times and even had Jabsco rebuilt one. It takes a major kit to do a proper rebuild +$250. It should be good as new, but a new pump is so sweet. There is a new pump on ebay for $449 w/free shipping.
http://www.ebay.com/itm/Jabsco-Pump-5320-0011-/140748506896#ht_500wt_1156One thing to remember is to not to overload the belt, the seawater pump's belt never slips. The pump only requires about 1/8hp of power to operate, verse the alternator which requires almost 4hp at 100amps.
My experience, the big things to check in overheating:
1) The Seawater pump. Like some have said, it's a positive displacement pump, if it's turning it's pumping water with a good impeller and cam plate. Inspect pump's impeller and cam.
2) Blockage (heat exchanger, seawater flow to the pump) When troubleshooting, break things down. Start at the thru-hull fitting. Shut the hull valve and disconnect intake hose and open the hull valve. Verify flow, it should rise about a foot above the valve. Shut valve and reconnect intake. Go to the seawater pump remove the discharge hose and disconnect hose leaving the heat exchanger. Now connect a hose and shoot water thru the heat exchanger. How does it flow, should be a nice stream. If it seems restricted remove Heat exchanger and take to a radiator shop. Have them rod it, clean and test. Or do a white vinegar flush of the system, it works for me every time. DO NOT INJECT WATER INTO THE SEAWATER PUMP DISCHARGE HOSE WITH EVERYTHING ELSE CONNECTED YOU CAN FLOOD THE MOTOR.
On my T-37, I don't have a seawater strainer and I don't have the oil cooler (oil to seawater heat exchanger). Best thing I ever did was remove that. It was done under the advisement of Marcus at TransAtlantic Diesel.
If you think it's the thermostat, watch the temp gauge/ While running if it stops and hangs around 185-200F for a while then goes up, my bet it's ok.