Cap'n Jacks' gets some bad reviews here on the DIS, but I eaten there twice in the last year (last Sept and this past May) and enjoyed it both times. We ate dinner there both times and lots of people were in shorts and t shirts.
Last Sept we had the tomato/buffalo cheese appetizer, crab cakes, steak & lobster x2 (yes, lobster was included during the free dining last year), and cheesecakes. In May we had a salad, crab cakes, seafood pasta, ribs, cheesecake, and key lime pie. All were good!
We have eaten there before. It wasn't the best meal we have had at Disney but it wasn't that bad either. We would eat there again if we were close by at a meal time but we wouldn't make a special trip there to eat.
We found the food pretty bland and portions small as well as expensive but then again if you dont enjoy it you would feel anything you paid for it too much..lol
We ate there once a few years ago. I'd agree with most of the others... its ok. We wouldn't avoid it, but we also wouldn't go out of our way to go back.
Shorts and t-shirts would be no problem at all there. Its pretty casual.