I was under the impression that my boat w/about 40 gal or gas in tank was going to be stored in a heated barn over the Ohio winter while upholstry work was being done

Well......it turned out to be an open air barn so now I am concerned about water in my fuel for initial start up on two rebuilt engines. I have never had to deal with water in fuel and I am always hearing about how initial start up and break in is critical to engine life, so should I drain all the fuel from the tank before starting........add some stabilizer and use it,......or just say the hell with it and do nothing

I'd drain it and start with some fresh fuel......just my .02

If you didn't add stabilizer, it is too late to add it now.
Drain the contents of your water separators and fill them with fresh gas and put them back on.

After the first 5 minutes of running, drain them and refill them again. It is helpful if you drain then into glass pickle jar so you can look for water blobs.

If its clean, then you're prolly ok.
If not, keep draining the separators every now and then in increasing intervals.

After you run the whole tank empty, refill and put on NEW separators.

Was the gas cap open? As long as everything was closed like it normaly would be you should have nothing to worry about. How long was the boat stored? If it was just the normal winter layover the gas should be OK even if you did not put stabilizer in it.

I have recently purchased a 302 Formula that had been sitting for 2 yrs with 70 galons of fuel. I put in 4 bottles of fuel stabilizer filled with 94 octane gas approx 30 gallons,checked for water.(in 5 gallons of fuel removed,there was only about 2 tablespoons of water) changed my seperators. I have burned through about 65 gallons of fuel now with only one or two higher RPM stumbles. Today I plan on changing the seperators again. Good luck I've been lucky so far! Andy