Weathertight means that in any sea conditions water will not penetrate into the ship, (ICLL).
A weathertight fitting shall have a strength at least equal to the bulkhead or deck in which it is fitted. In practical terms, the difference between weathertight and watertight is that a weatertight fitting shall withstand water pressure from the outside (be watertight from outside), whilst a watertight fitting shall withstand water pressure from both sides.