If yur engine is "fresh water cooled" which is common for anything over about 10 HP, the fresh water is not really water, but is a mixture of water and antifreeze. Since you said there is a reservior, chances are that is what you have. The sea water goes thru a heat exchanger, to cool that antifreeze/water mix, just like an auto radiator is cooled by air flow thru it. The heat exchanger is a honeycomb with lots of little tubes to transfer the heat from the engine coolant to the sea water. If the
pump impeller is a couple years old, pieces of it can break off and plug up the heat exchanger, causing overheating. sometimes it doesn't affect the flow of sea water significantly, so you think you have cooling, but if the engine coolant is not flowing freely, it overheats! If you have not changed your impeller for two years, do it now. If there are any pieces of rubber missing from the old impeller, take your heat exchanger apart and clean it. BTW, before you start your engine EVERY DAY, check the oil! If you change oil every 100-150 hours, which most engine manufacturers reccommend, the oil should not be below the dipstick unless there is a leak. Checking oil level every day you use your engine will let you know if there is a leak. It can happen any time, due to the inherent vibration in a boat.