Thanks DrJudyB. Yes I'm sure it's open cell. It's white like a styrofoam cup and little beads break off easily. The Mac25s came with that kind of foam.Are you sure the flotation foam in your Mac25 is open cell? Open cell foam is typically used only for interior settee and vberth cushions because it's soft and squishy.
From my experience with building science, once you get up to about 4" of closed cell foam they start to refer to what you've created as a "vapor barrier" since the vapor diffusion potential is almost zero. I'm not sure how vapor diffusion relates to submersion, but I suspect it's related. I've emailed the manufacturer to see if they have an answer. Probably $300 vs. $65 seems like chump change to some but our budget is pretty constrained so every $100 is a hard choice.IMO, trying to seal open cell foam by surrounding it with closed cell foam is not a good idea Remove it and replace it with something else if you want reliable flotation. Even a net full of empty water bottles that is secured to the boat would be less prone to failure. Or buy the marine grade stuff. Or use pink foam or styrofoam, but inspect it regularly because it's not reliable over the long term.
I'm a relatively new sailor in terms of years, but I've spent hundreds of hours out on the water, including lots of lessons with more experienced folks. I don't plan to cross any oceans, but I probably will cross Lake Michigan and/or go to the Bahamas one day. My concern is that the Mac25 is pretty thin fiberglass and could be holed more easily than a bluewater boat. Or the swing keel could break away. So I want to be careful and try to keep the positive flotation if I can.