I agree with you 100%! Which is why I drilled/am keeping a drainage hole in the plastic "plug" at the bottom of the tube.
However, and to pass this on... when I spoke to the Hobie dealer about this (water getting into the tubes/corrosion), they said "the wings are supposed to be sealed watertight." So from the factory, they are assuming them to be watertight.
When I originally took the wings apart, there was so much sealant between the plug and the "wood dowling", that I had a heck of a time just getting the plug out, so obviously Hobie did seal the plugs in (and I have pictures to prove it). Also, I was going to replace the castings that hold the inner bar, and when I picked up the rivets (from the dealer), I was also instructed to use those "sheaths" that go over the pop rivet, and into the drilled hole, before riveting... the goal here they said, to seal off the unit entirely from water getting in.
This was my experience, and maybe the plugs were just sealed for that year... but as for the casting rivets, that's what I was told just weeks ago.
But as this has been my experience, I am not going to seal the tubes entirely, as I don't want the problem to happen again. I had always assumed, that aluminum does not corrode in fresh water (and this has been a fresh water boat). However, my neighbor, a retired pilot (from float planes to 747's), told me otherwise... aluminum can corrode in fresh water, if it's trapped in. I believe Hobie may have changed the wood dowling, to a wood box, that will not allow water to be trapped against the inside tube wall of the wings... which is obviously a step in the right direction. And for all I know, it could be the aluminum used for these tubes, wasn't quite right to begin with, as some of the corrosion spots, are well above where the dowling was put in. So I don't know the answer.
I had posted this question to get some real world advice, and to alert others to what my experiences were, and what I learned. I send thanks to you, for your reply.