I also have a PSC, and have recently done alot of work to my aft water tank.
I have an Orion, and the aft tank is under the sea berth in the port quarter, I think yours would be aft of the galley under the cockpit on the port side, correct? Anyway, my aft tank vent had began leaking, and to repair the leak I had to remove the lid. My tank lids are plywood with a hard water proof coating on the underside (can't remember the trade name for this board right now), the face that is in the watertank. Mine also have clear screw on hatches that are about 6 inchs across for tank cleaning and accessability. I had use a razor knife to work all the way around the seam for the hatch, and then slowly go around the edges prying the lid off with acouple of thin pry bars and flat head screw drivers. I went really slowly so as not to delaminate the plywood cover. If its just the cover that is leaking , then you should remove ALL of the old silicone, so you have good clean faces when you go to rebed the lid. Apply planty of new silicone and clean up the excess that oozes out as you go. I left some heavy items around the lids rim after reinstalling so that the lid was well set in place whille the silicone set.
If you don't have the same type of access hatches that I have,or something similar, it is something to consider installing once you get the tank lid off. They are about 6, maybe 8 inches across, and they make cleaning and inspecting the tanks a breeze.
I do not have full plexiglass covers with plywood inserts like you do, but I imagine that the lid removal procedure would be about the same for both of us though. Rebedding the whole thing would cure the leak I bet.