When I bought my 2000 Seafarer a couple years ago, the pop out pie plates would fall out from wind, water hose, or just choppy water. It must have been an ongoing problem, because the previous owner had left several packs of new ones in the drawer under the seat. Replacing them did nothing to improve their hold.
I removed the O-rings, put a number 64 rubber band in the groove as a spacer, and put the O-ring back on top of it. It gave the plate a very snug fit, that you had to press in firmly in order to seat it. I have not had one pop out since.