I'm surprised no one remembers this from David Leaf's booK (pgs. 143-144):
"The vocal arrangements on Surf's Up weren't as full as those on Sunflower either, and that is mostly because Brian wasn't working on the album. Bruce recalls, 'It was strange to be doing vocal arrangements to make it sound like the Beach Boys when we were the Beach Boys. That's a little weird to me.'
"The resurrection of the song 'Surf's Up' is a confusing tale. Part of the album track was from a recording Brian had made for the 1967 Leonard Bernstein special; other segments were recorded in 1971 to fit in with the old track. Bruce: 'I remember thinking, "Well, if I voice this chord into Brian's part from the end of Carl's part, it'll sound okay and no one will know about it." We ended up doing vocals to sort of emulate ourselves without Brian Wilson, which was kind of silly.'"
"The vocal arrangements on Surf's Up weren't as full as those on Sunflower either, and that is mostly because Brian wasn't working on the album. Bruce recalls, 'It was strange to be doing vocal arrangements to make it sound like the Beach Boys when we were the Beach Boys. That's a little weird to me.'
"The resurrection of the song 'Surf's Up' is a confusing tale. Part of the album track was from a recording Brian had made for the 1967 Leonard Bernstein special; other segments were recorded in 1971 to fit in with the old track. Bruce: 'I remember thinking, "Well, if I voice this chord into Brian's part from the end of Carl's part, it'll sound okay and no one will know about it." We ended up doing vocals to sort of emulate ourselves without Brian Wilson, which was kind of silly.'"
David didn't have access to Steve Desper or Carl at the time he wrote the first edition of his book, so the details of how "Surf's Up" was resurrected did not surface until later. Bruce's comment actually references two parallel situations--first, the specific need to create a vocal arrangement for the tag of "Surf's Up" that had sufficient continuity to sound organic and not--for lack of a better term--"retrofitted"; second, the general tendency for the group to record backing vocals without Brian in the mix, an occurrence that became more frequent during Brian's reclusive years.
And Desper did make it clear in a thread here awhile back that Bruce was often the go-to guy for these types of vocal arrangements, particularly when Brian was absent.
But from Bruce's description above, I have to wonder whether he was actually referring to the little vocal bridge between the "Brother John" vocal into the "Dove-nested towers" section...because that is where we really go from Carl to Brian in the '71 version of the song.