What would a submarine do in space?

Write in the sky:"Josh is an imbecile".

Sci-Fi author Harry Harrison did that in a book (the Daleth Effect). An inventor came up with antigravity so had no trouble lifting tons into space, and there was this handy submarine belonging to the Danish Navy.

It seems plausible at least - the submarine was designed to take a pressure differential, and had life-support equipment for a sealed environment. A vacuum would be the same pressure differential as a mere 32 foot of water, and an inch-thick steel hull would do a better job of blocking radiation and micro-meteorites than a a shuttle body.

(I thought the Yellow Submarine was just a music video, not that the Beatles really did live in it)

Sink pretty fast, unless it got a pair of rocket to keep it afloat. Other than that, it's useless as a space vehicle, it was design to withstand pressure from the outside, not from the inside.

You can visit Lucy in the Sky with diamonds.

Just float around