My family's had a home in Door County for 20 years (and was going up there well before that) so I've had my share of fish boils. The White Gull is great (DISCLOSURE: I know one of the owners' daughters) and you won't get a bad fish boil there. The setting is great and the pie is too (as you can see from the above picture).
Having said that, it's expensive and really, a fish boil is just the same 3 ingredients (whitefish, potatoes, onions) dumped into a boiling pot of water. There's little-to-no variation among the quality of taste of the actual "hot stuff" of most fish boils (although the accouterments--cherry pie, bread, options like chicken instead of or in addition to the fish--might vary in quality), so unless you really want to pay for atmosphere, I'd suggest that you'll be just as satisfied with the quality of the food someplace less expensive. It's certainly worth the experience, at least once, but don't expect to be bowled over.
In addition to the White Gull, the four others I'd recommend:
Old Post Office (Ephraim)
Sandpiper (Bailey's Harbor)
The Viking (Ellison Bay)
Northern Grill (Sister Bay)
Check beforehand to see if they're serving--after Labor Day, a lot of these places cut back.
Homer: Oh, God, why do you mock me?
Marge: Homer, that's not God. That's a waffle that Bart threw on the ceiling.
Homer [contemplatively, to waffle]: I know I should not eat thee, but...[takes bite]
...Mmmmmm. Sacra-licious.