>the propeller on the back of it isn’t part of the “body” and the “body” of a submarine is rigid and immobile.

That's a choice to limit the meaning of the term to the rigid/immobile parts of the external boundary of an object. It's not obviously the correct choice. Presumably you don't take issue with people saying planes fly. The issue of submarines swimming seems analogous.

>Is this an intellectual exercise for you or have you ever in your life heard someone say something like “the submarine swam through the water”?

I don't think I've ever had a discussion about submarines with anyone, outside of the OceanGate disaster. But this whole approach to the issue seems misguided. With terms like this we should ask what the purpose behind the term is, i.e. it's intension (the concept), not the incidental extension of the term (the collection of things it applies to at some point in time). When we refer to something swimming, we mean that it is moving through water under its own power. The reference to "body" is incidental.