Any particular antifreeze recommendations for winterizing 496 HO sea water system?

-50F pink stuff from just about anywhere is fine; WalMart, TractorSupply, Lowe's, etc.

I like the brands with the rust inhibitors as well, like the purple version at West Marine. The pink stuff usually is just for freeze protection, no added corrosion protection.

Test the coolant in the closed side while you are at it. Just for peace of mind.

If there is any water in your block, it will dilute the -50 stuff. The -50 rating is not when it freezes, that's when it will burst a copper pipe if it's undiluted. I use the -100 with corrosion inhibitor and I store in a heated garage. If my furnace goes out, I'm not taking any chances. Read the fine print on that stuff. I've used the -50 when I winterized my in-laws cottage before I knew better. I blew the lines out and then pumped the lines full of anti-freeze and their PVC lines still burst. Of course we get to -20 pretty regularly.

We use the Merc -100 Non-Tox and drain everything then refill complete (sea water side in your case).... springtime, just attach the battery cables and go.

I seen a tow boat that had silt in the block and trapped water above the block drains.. -100 can't overcome that so make sure when you drain, water does come out in quantity!

Definitely check the antifreeze in the CC side for its freeze point... or change out if it's been a few years.

Quoted for Truth. Beware of using the standard "water line winterizer', it actually freezes at 7 degrees...regardless of the boast of '-50'. STARBRIGHT has a pink that is made for engines, as does West Marine, and of course the purple stuff from West Marine is even better.

We use starbrite -100. Costs more but it’s piece of mind. We are in heated storage too.

I've been using the -60 Purple West Marine stuff for years.