English expressions about transportation

Hello I am English as a second language learner.

I have few questions about English expressoins about transportation.

Like..I wanna say that I go to school by bus.

"I go to school by bus"

does it make sense? and can I say "I go to school on bus" ?

I was watching 17th episode of the 3rd season of the tv show "LOST".
I just heard that Hurley says "We gotta go out there" and then Charley says "In what!?"

I understood that Charley was asking which one would they use to go out in the sea. (can I say go out in the sea btw?? )

hmm...can I say "by what?" insteand of saying "In what?" ??

and I have written some sentences to ask about this one..

1. Let's go to school by bus ( no "a" bus or "the" bus right?)

2. Let's go to school on foot
( I mean Let's walk to school, in this case you say "ON FOOT" right?)

3. I go to school in a car.
4. I go to school in my friend's car
5. Let's go to school in my car.
6. Let's go to school in car.

do these sentence make sense? especially the last 4 sentences.
From number 3 to number 6.

please answer my question and thanks for reading!