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The 16SL has a bizarre drainage system, where it is supposed to run off the deck, onto the top of the tank, then into the stern port well through that small tube. (If that well fills up, say when at mooring after a lot of rain, excess water then flows out the scuppers.) This means there's always some moisture on the tank top, but if the drain is working properly not too much (when I bought my 16SL the tank cavity was full of dead leaves, and the drain was pretty much plugged). As Al notes, when you're not using the boat it's a good idea to pull the inspection plates to get some air flow. I sometimes use a small fan to help.

In any case, as much as I hate to drill unnecessary holes, I think your best option is to drill a new drain hole below the old one, and epoxy in a length of pvc, just like the factory did. Make sure it's at the level of the tank top and then you'll have as good drainage as you're going to get.

John