Engine antifreeze is mostly ethylene glycol with small amounts of anticorrosion and other additives. EG is extremely toxic. Don't ever, ever use it to winterize your boat's potable water system. And since it all gets dumped back into the ocean eventually, don't use it for anything other than your engine's coolant system.
RV/Boat antifreeze- the pink stuff is propylene glycol, water and a bit of additive- not sure what. PG is non toxic and is the same stuff that you drink before a colonoscopy!!!!
Use this for all winterization: engine raw water, potable water, air conditioner raw water, deck washdown raw water, etc.
It comes in two forms. By far the most common is prediluted gallon jugs that is good to a -50 burst temp. This means that it will start to freeze at that temp, get slushy, but won't expand and burst a pipe or fitting. I haven't checked the data for higher dilutions but I suspect that if you were to dilute this stuff 50/50 with water you would be ok to zero deg F.
The other form of RV/Boat antifreeze is typically sold in 30 oz containers (about 1/4 gallon) and is almost pure propylene glycol. You can and should dilute it with about 3/4 gallon of water to make the same as the common stuff before using.
When I use the prediluted -50 deg pink stuff, if I get a strong pink color out of the exhaust, faucet or whatever I believe I am good.
Boat yards do charge an astronomical sum to winterize your boat. I can do my engine; the potable water system with two sinks, showers- internal and external and ice maker; the air conditioner; all in about an hour or two with 5 gallons of antifreeze. At full yard rates of $100 per hour and WM pink stuff prices this comes to no more than $225, plus a few reusable hoses, buckets and fittings.
David