Difference between watertight and weather tight with reference to vessel?

Answer 1

"Watertight means designed and constructed to withstand a static head of water without any leakage" (46 CFR 114.400 and 175.400). This refers to structures that must withstand hydrostatic loading for extended periods of time, including the hull bottom and sides, main deck plating, tanks, and "subdivision bulkheads". The latter are meant to contain the spread of water that enters the hull as a result of hull breach due to collision, grounding, etc. All weldments and penetrations of such structures are subject to strict testing requirements to ensure that no leakage occurs under the particular design conditions of the vessel.

"Weathertight means that water will not penetrate in any sea condition" (46 CFR 114.400 and 175.400). This refers generally to topside structures subject to occasional and short exposure to water on deck, from boarding waves or wind driven rain and spray. Regulations specify arrangement and height for weathertight appurtenances, as well as load and welding specifications, which are generally less stringent than for watertight structures.