My interpretation has always been while others may have gone surfing, Dennis 'surfed'. Much the same as I might write a song, but Brian writes a 'song'. He knows what he is doing and is good at it!
You are absolutely right, Mr. Funky. It is unquestionable that Dennis was the only competent surfer & the rest of The BBs - amateurs. For example, I have a collection of my own goosebumps written in 12 years old, but there's no way I'm a writer, let alone acclaimed. So, the "Only one of the Beach Boys actually surfed" statement is strongly valid.
Speaking of Bruce, well, I doubt he is as good a surfer as Dennis, plus he's not an original BB, therefore, can't be counted.