Good question . . . I would suspect RV/Marine antifreeze to have a lower freeze point than washer fluid and is non-toxic, since it''s developed for use in pressurized onboard water systems . . . not that that matters any with a boat engine. However, all boaters should ensure that whatever is added to fill the engine block over the winter, is non-toxic. Good for the environment & safe for the critters & pets that may consume it.
Aside from that, there dosen''t appear to be any other advantages over washer fluid.
I only use marine engine, non-toxic stuff in my diesel''s raw water section, since it possesses anti-corrosion ingredients. The cost is only a couple dollars more per gallon . . . substitutes are false economy. Using anything else misses the point of adding anything at all. Just completely drain all the sea water if you''re that cheap.