Cryptic Themes in "Octopus's Garden"

When I picked up my brothers from cross-country practice today, we had the following discussion in the car: 

Me: [singing] "I'd like to be....under the sea....in an octopus's garden in the shade..."

Thomas; That song makes no sense. 

Isaac: Does it have to make sense?

Thomas: I'm just saying-I mean, the first two lines make sense..."I'd like to be...under the sea," but the third part-

Me: It's a simile. 

Isaac: It's not a simile; he didn't say "like" or "as."

Me: I mean a metaphor. It's a metaphor. 

Thomas: Wait-we got to analyze this. "I'd like to be...under the sea-"

Me: Why is it "under the sea?" Why not just "in the sea?" 'cause if it's under the sea, you're like, in the ground. 

Isaac: It's like saying "under a tree," like you're under the leaves in the shade, not literally underneath all the roots and stuff. 

Thomas: That's not the metaphor though. 

Isaac: No, it's not a metaphor, it's just bad grammar. 

Me: No, the metaphor is the garden. They're just saying 'garden' instead of....'yard' or whatever it is octopus's have. 

Thomas: Yeah, the garden is a metaphor for-

Me: It's his food-source. 

Thomas: Yeah, so he wants to be in the octopus's food source, so he wants to be eaten by the octopus. 

Me: It's about suicide! 

Isaac: Wha-no it's not! 

Thomas: Yeah! Isaac, look, it's a guy and he's being bullied, so he wants to commit suicide and feed himself to the octopus.

Me: Oh-wait, no! It's "in the shade," so it's like humans are like drugs to octopuses and the octopuses do human deals like drug deals, cause it's "shady business" and the octopus does it "in the shade."

Thomas: Yeah, and he wants to be an octopus drug, 'cause it'll make him important-

Me: Like in Shawshank Redemption! Like when they were saying Brooks was an important man in prison and nobody outisde prison. 

Isaac: What? 

Thomas: Isaac, it's like there's a guy who's a nobody, and he wants to go in the ocean 'cause he'll be valuable to the octopuses.