Why is swimming or being submerged in water typically bad for you after back surgery?

Wound. Initially after surgery you want to avoid submersion in a pool or tub due to wound healing. You want to avoid it due to the possibility you may infect the wound. Once the wound has healed, swimming and pool therapy is often employed to help with rehab after major back surgery.
It is only to be. Avoided until the surgical incision site (s) have fully healed which in most individuals that is usually after 3 weeks. Otherwise, it could affect wound healing or even lead to an infection.