I'm listening to it as I'm posting this and I think you setting expectations way too high by putting it up there with "God Only Knows" which is their best song and one of the best songs of all time. That's not to say "Surf's Up" is bad, in fact it's quite good, but it doesn't approach "God Only Knows".
I just saw this post now! Late to the party, I guess.
I've been listening to the song "Surf's Up" for years as it's been released previously (it's actually the title track of their 1971 album). While I realize it does not have the cultural impact of "God Only Knows," to my personal tastes, I think it's as good, and this is coming from somebody who's lived with the song for a while. They're my two favorite Beach Boys songs.