Alright, I might as well respond to my own post and share what I discovered in changing my coolant.
To do: Remove the filler cap on the engine with the engine cold. As you face the engine, looking aft, locate the large rubber hose on the front of the engine that runs from the freshwater pump to the heat exchanger. Find the low point on that hose, and there's a drain. It opens with an allen wrench. Turn it clockwise until it's loose, then hold a drain pan under it while you totally remove the plug. Let it drain into the pan. If your pan's too small, just plug the flow with your finger as you empty your pan into a larger container. Continue until it stops flowing. Replace the plug and loosely tighten it.
On the top of the engine toward the front are the two hoses that run to the water heater. They're right near water pump. Remove one of them at the engine. I did the one on my left, but either's okay. Put the tube in your mouth and blow into it forcefully to empty coolant from the water heater lines. And...while blowing on it, use your finger to plug the fitting that you removed the hose from. You'll hear gurgling and the coolant will run into the heat exchanger. Oh, and avoid getting coolant in your mouth, eh? After four or five good toots on the hose, remove the plug from the low point in the hose again, and drain it into your pan again. Replace the plug and tighten it.
The other plug is on the oil filter side of the engine, just a couple inches aft of the oil filter. The good news: when you drain this plug, not a lot comes out. The bad news: I don't know how the heck you're going to get to it. In my boat, I've removed the aft cabin door and the wall that's next to the engine cover, giving me total access to the engine from three sides. I highly recommend this mod, but it's not for everyone. Shy of that, I'd suggest putting a deck plate, round or rectangular, in the wall. The size of a dinner plate would be about right. This will give you access to the drain plug and BONUS...it will give you very good access to the oil filter, so you can actually clean up the oil that drips down the side of the engine when you change it.
So, if you have access to the second drain, you place a socket wrench on the bolt, which opens the drain. There will likely be a vinyl tube running from this drain into the fiberglass pan under the engine. You can direct the tube into your drain pan, turn the bolt clockwise, and the coolant from the block will run into your pan. Put the filler cap on, use your finger to plug the fitting on the top of the engine again, and blow on the hose again to force more coolant to run out the drain plug. Now you've removed about as much coolant as is possible. Use the socket wrench to close the block drain...easy does it. Re-attach the water heater hose to the fitting on the top of the engine.
The coolant overflow cannister has to be emptied. Just pry its cap off and tug the cannister straight up to remove it from the bulkhead. Empty it completely. Replace the cannister.
At this point, you could simply refill the engine with coolant. Use the approved coolant of your choice. I like the premixed 50/50 stuff from Prestone (it's a Dexcool clone). Once you've filled as much as you can at the filler cap on the engine, fill the overflow cannister to halfway between the high and low points and replace it's cap. Put the filler cap back on the engine.
You're ready to run the engine. Let it warm up, and watch for any leaks. Add any coolant if necessary to keep the cannister level at halfway between the level marks. Next time you take the boat out, watch your temp gauge, and after running for a while, shut it down, open the engine hatch, and add coolant as needed.
If you wanted to flush your engine after you removed the coolant, don't refill the engine with coolant after you've drained it. Instead, fill it with water, just like you'd fill it with coolant. Run the engine until it's warm so the thermostat opens up.
Let the engine cool and then drain the water per the above procedure.
Refill the engine with water and radiator flush liquid (I used Peak brand...Prestone's good, too). Run the engine to warm it and open the thermostat again. I ran mine for about 30 minutes. Let it cool and drain it again.
Refill with water. Run it until it's warm and the thermostat opens up. Let it cool and then drain the engine again.
FYI: I found it takes about 45 minutes for the engine to cool enough. You don't want it so hot the you get burned by the hot coolant.
Then fill with coolant as per the above instructions.
Confession: I skipped one of the steps. I drained the old coolant, and skipped the next step of running the engine with pure water. I went directly to running the engine with the water and Peak system flush. Followed by a water flush and refilling with coolant. I say, "good enough". Some folks would do two water flushes after using the flush stuff. Not me.
If you were unable to get to the drain plug on the oil filter side of the engine, it's probably not that big a deal, but if I hadn't removed my wall, I would definitely put a hatch plate in the wall so I'd have better access. 'Sup to you.
I'd say changing the coolant's more hassle than changing the oil, but I'm sure you'll find worse chores on your boat. It would go quickly if not for the intervals of getting the engine hot and cooling it down again repeatedly. I put the boat in reverse and let her tug on the dock lines while I ran at around 1600 to 1800 rpm, just to speed the warming. I never got it to come to full temp at the dock, but it got warm enough.
