Unfortunately, it seems the OP didn't research fully before booking their cruise if letting their little one get wet was important to them.
Unfortunately, at least on cruises out of the US, it's a health issue. If there were an accident in the pool, it would need to be closed, cleaned, sanitized, and refilled, closing the pool for hours. The ships that have splash pads for swim-diapered kids have them as a separate system. Just to note, this rule doesn't just apply to babies, but to people on the other end of the spectrum who need to wear similar items.