These are the places I like......
For ultra-casual: I like the Tiki bar/hut in Riviera Beach Marina:
Tiki Waterfront Sea Grill - Google Maps
As someone already mentioned, Sailfish Marina is decent, though I like the Tiki bar/hut better
There is also a new rum bar in the little shopping center that contains Wings on Singer Island: I don't recall the name, but Wings is on the north end of the retail center and the rum bar is on the south. I've been there twice for lunch: service was a bit slow, but my food was good.
If you are going up to Jupiter: I like this place
Guanabanas - Jupiter Tiki Bar and Restaurant not the cheapest place it town, but a fun atmosphere if it is not going to rain or if it is not too hot. The fish tacos and burritos are #1 in my book. Kids love this spot since it feels like being in Hawaii
For ultra ultra ultra casual in Jupiter, I like Dune Dog
Dune Dog cheapest dolphin (mahi) sandwich in town
For a good, but slightly over priced burger: try Grease in downtown WPB:
Big Time Restaurant Group Clematis street also has a bunch of other places that are decent like Roco's Tacos and Bradelys