Even closed cell foam will absorb water if it is immersed long enough. (E.g. The hull is left with water in it for long periods of time! For example, look at some of the foam blocks used for floatation at an old boat ramp) But its not the styrafoam blocks that is the real culprit here, it’s the two-part expanding foam used to keep the styrafoam blocks in place! If water is left in the hull for a period of time (exactly how long I do not know), the expanding foam will turn into a "sponge" and you can literally squeeze the water out of it! The up side of this is that "what goes in easy, will come out easy". This is why putting a fan, heater, shop-vac, etc, over an inspection port for a couple of months will drive out most of the water. But not all, as the closed cell foam does not out-gas the water very quickly.
I think that Tap Plastics can sell you a good grade of styrafoam which you could then saw up to fit into the inspection port in front of the mast. Then you could use the two part expanding foam to "Glue the whole mess back in place". It will not be pritty, but I don't think anybody will be looking in you inspection port anyway. ;-)
The foam is similar to those cheap styrofoam coolers. All foam will absorb water, closed cell will take it on at a much slower rate. Open cell is like a sponge, nuff said. If your Sunfish is heavy and you can not see/hear any water, the water is in the foam. Best method to dry your boat out is by installing inspection ports and getting warm, dry air inside. Getting water out of the foam takes a bunch of time (possibly months)
In response to Supercub's question, I just bought the boat and I noticed the deck flexing a lot behind the bow inspection port. After looking at Wind Line's "How To" diagrams I saw that a block of foam was absent. I am now questioning the seller to see if he removed it for some reason. Any opinion on how serious it would be to not replace it?
If you plan on racing, you may have a problem. ISCA Rules, part 3.1.3, states that the foam may not be removed. To replace the foam you will need to get a block of foam about 4" wide x 12" tall x 2' long. It will taper towards the bow. You want it to fit fairly snug between the deck and hull. Cut it like a loaf of bread (a serated bread knife or "hot wire" will cut the foam) to fit thru the port and starting at the mast step and start stacking it back together. "Foam in a can" or a foam compatabile glue can be used to reassemble the block. The "foam in a can" can be used to secure the reassembled block in place in the hull. While you have easy access to the bow, you may want to thru bolt the bow handle and/or the bullseye/pully and halyard cleat. Be sure to use 3M 4200 or Marine silicon under the fixture to seal out any water. Good Luck.