I did a search but did not find a topic covering vocals-only versions of Beach Boys songs.
There is nothing that makes me appreciate Beach Boys songs more than hearing them with the music removed.
The music as a whole is amazing and the instrumentals are terrific as well, but face it - if you are a Beach Boys fan, you are probably a fan of their harmonies. For me, hearing a song you have heard a million times stripped of instruments brings it to a new level that is so emotional and spiritual. Often imitated, never duplicated. The original Beach Boys vocal stack (including Bruce) is untouchable.
Here are some of my favorites:
Wouldn't It Be Nice
California Girls
Slip On Through (the droning harmonies are awesome)
God Please Let Us Go On This Way (snippets)
Help Me Rhonda
Kiss Me Baby
Please Let Me Wonder (all over the place with subtle changes, but perfect!)
Sloop John B
Break Away
'Til I Die
God Only Knows (including the "How was that? Was that cool?" line.
)