I don’t know where you’re from, but the vast majority of cars in the US have hollow doors. The only exceptions might be expensive luxury cars. There is no foam in them. If you poured milk in them it would most likely drain out of small holes in the bottom, although it might splash & stick to some of the internal components and make a smell.
The more you move it around, the longer it will take to fully heal. If you were to immobilize the joint with an athletic wrap/tape, it would probably be fine. Without that, I’d imagine it would be hard to keep it from moving while swimming.