Stillraining gave a lot of good information and advice but I thought I'd add just a bit here. Had a 2GMF Yanmar on my old Hunter 31 that overheated a few times and here is what I learned:
1. DO NOT IMMEDIATELY SHUT DOWN THE ENGINE UNLESS YOU REALLY HEAR SOME BAD (GRINDING) NOISES. Remember, when all internal combustion engines are shut down, the core temperature actually RISES (known as "heat soakback)! Why? Think of why you really start to sweat when you first get off a bike or complete a run. What little cooling you may have had is now gone. What you should do is reduce power to idle and wait a minute or two, then shut it down. That will allow the block to cool naturally and dramatically reduce heat soakback. You can do even more damage to the engine by immediately shutting down an overheated engine!
2. Start at the intake seacock and work your way all the way through the engine to the water lift muffler to determine the reason for the overheating. If there is a problem in the SW cooling side, you will see a drop in water flow out the exhaust
. The primary culprits are: debris in the intake, clogged SW filter, bad impeller, clogged heat exchanger and a clogged mixing elbow. I'd take my time and work my way one step at a time. I had a situation once when I'd changed the intake hose on the "vaccum" SW side from the filter to SW pump
. Used the wrong kind of hose and every time I ran the engine higher than 1500 rpm, the hose collapsed and the engine overheated. When I shut it down to check the problem, couldn't find anything wrong! Had a mechanic take a look and he figured it out in about 5 seconds. Best $50 I ever spent on that boat!
3. If SW flow is not a problem, it has to be in the FW side and unless you are using too "strong" an antifreeze mixture it is almost certain to be either the thermostat or clogging in the FW side of the heat exchanger.
If you saw smoke in the engine compartment, that isn't a good sign but could be the heat burning off engine paint
or something else like that. I would definitely change the oil (and it wouldn't hurt to save a sample and have it analyzed. Matter of fact, that is probably one of the best things you can do since you will get all sorts of valuable info on the condition of you engine) and make sure the oil isn't milky or smells burned. I'd also make sure you don't have any leaking seals (head gasket, etc.) which could have been compromised in a severe overheating.
S/V "Magic" C400