I was wondering if we had two things going on here. The welding of the plate is secondary to the weed hatch leaking and the ingress of water was down to a loose or poorly sealed weed hatch. The welding just needs doing but wasn’t the cause of the water in the bilge.

These are the type that are often fitted at motorhome aires in the UK or in car parks by local councils. They do usually have a rinse tap though!