I had the same problem with the hatches on my old Sailfish. In my opinion, it's the way they are built that causes them to crack. Now, I could be wrong here, but I believe I remember reading somewhere that the hatches are built using some sort of injection molding process. I don't remember them being a traditional layup with glass mat, cloth, etc. and resin, it looked more like a putty type material with glass fibers mixed in.
What I would do is open up each crack on the finished side and then fill them with a cheap putty like bondo. Once that cures, sand the patched area smooth and sand off all the nonskid. Take it down to a pretty fine sandpaper. 400 wet would be ok, but the finer you go, the better the NEW hatch will look. Then wax the entire part with a mold release wax and finish it off with a light coating (no runs) of PVA applied with a Preval hand held sprayer. Next, layup several layers of glass over your "repaired" hatch and let it cure. Once it's hardened up, pop your mold from teh original hatch. Clean the mold, of any wax and PVA, and then lightly sand it (if necessary) to remove any blemishes. Wax the mold and PVA it. Paint or spray on new non-waxed gelcoat, let that tack up and they lay new glass w/ poly resin. Let the new glass tack up. Put in new core piece and weight it down while the bedding compound (resin and cabosil) tacks up. Then cover with a few more layers of glass and once that starts to harded up top it off with gelcoat (waxed or spray PVA over it to facilitate full cure). Once cured, pop it from the mold and reapply the non-skid based on one of my HOW TO posts at the top of the TIPS board.
You glass weight will vary depending on the radiuses you need to deal with. Heavier stuff doesn't like to bend. Number of layers to build proper thickness depends on the weight.
Lots of work, but if done right you will end up with indestructible hatches that are better than new.
