If they were installed "right"....they may have been bedded with something like 5200 which can last forever on GRP. You may have to chew them out. Once they are out, you bevel the glass out to a 1:12 slope (or 1:18 if you're really conservative) on both sides, and start laying in new material in overlapping layers to fill the hole. Oh, and if you want perfection, of course you'll be vacuum-bagging those.<G>
Tedious, but that will bring the patch/plug up to full strength with the hull
(with or without the vacuum bagging!)
Sometimes using a hacksaw or Sawzall to cut the fitting in two (insert blade in hull and slice outwards) and then collapsing the fitting is the easiest way to do it. Sometimes, with a long crowbar or pipe extension, you can actually unscrew them and pop them out. But not if they were bedded with 5200.<G>
A sea chest is a nice idea, but considering all the work...I think I'd rather install a more robust bilge
pump/alarm and buy a pack of damage control plugs.