The career of the Beach Boys started with songs about surfing, beaches, babes and cars. They were popular for their singles but the albums were in most cases based around the singles with some fillers.
Sloop John B single cover
The albums "The Beach Boys Today!" and "Summer Days (And Summer Nights)" released in the first half of 1965 ended that period and their music heavily progressed. And as many artists in the late 60's some band members started experimenting with drugs. That’s part of the game I guess.

Finally in 1966 "Pet Sounds" was released. An album fully arranged, produced and composed (with Tony Asher) by Brian Wilson. Although the long recording sessions for this album was something the band except for Brian Wilson didn't like. Brian was inspired to do it this way by Phil Spector's productions and hired Spector's studio musicians for the recording of "Pet Sounds". But the result is the best album the band recorded in their career. A weird choice for the album cover is a photo shoot at San Diego Zoo but that didn't prevent the album from becoming famous. The Beatles did it better with their Sgt. Pepper’s album cover with the pop art from Peter Blake.
Brian Wilson decided not to go on tour anymore and only work in the studio with the band. Also in example to the Beatles who stopped touring in the second half of the 60's and became a studio only band.
God Only Knows/Wouldn't It Be Nice single cover
Al Jardine suggested to Brian Wilson to use the song  "Sloop John B/Wreck of the John B” a traditional Caribbean folk song. The song was covered by many artists before like The Kingston Trio, Johnny Cash and the Weavers. It became an immediate worldwide hit. After hearing "Rubber Soul" from the Beatles Brian was destined to make such an album himself. In answer to this the Beatles were inspired by "Pet Sounds" to release their "Sgt. Pepper's Lonely Hearts Club Band". The album consisted mainly of introspective songs like "You Still Believe in Me" about faithfulness, "I Know There's an Answer" about LSD users and "I Just Wasn't Made for These Times", an autobiographical statement on social alienation.
Original album cover
Pet Sounds
Wouldn't It Be Nice / You Still Believe In Me / That's Not Me / Don't Talk (Put Your Head On My Shoulder) / I'm Waiting For The Day / Let's Go Away For Awhile / Sloop John B / God Only Knows / I Know There's An Answer / Here Today / I Just Wasn't Made For These Times / Pet Sounds / Caroline, No

Sloop John B

 I Just Wasn't Made For These Times

The Weavers version of The Wreck Of The John B