Pretty scary on the sailboat going turtle.. My previous sailboat was a Catalina 16 (similar to the 18 - fixed wing keel) and one time I got hit with a big gust and the boat rounded up into the wind with about 50 gallons of water comming into the cockpit area. No problem, all the water drained out quickly and the boat stabilized pointing into wind (scared the family a bit).
On the 26S, in all the torture and dumb things Ive done with it (including roundups in big gusts), I also never had any water come into the cockpit. Its probably a good thing no water has come in because of the below:
I used to keep my gas tank right on top of the drain. I moved the gas tank but last season, I watched a water bottle plastic cap (only a bit smaller than the drain hole) drop into the drain. I did some back flushing this spring but for all I know, the bottle cap is still in the line so Ill probably have to take the hoses apart to make sure. I also put a screen on the drain hole now to keep junk like bottle caps from plugging the drain up.