Inboard vents are definitely forbidden in out local regs, and the outboard vent is of a minimum diameter and shut off valves are also forbidden on that.Water in your tank comes from a leaky deck fill or most commonly the tank vent line
inspect your tank fill..rubber O ring and design ..some are garbage
on custom builds the … vacuming in moisture thru the tank vent problem … is solved with two tank vents
one large diameter refueling vent, vented overboard , with a valve labeled ….CLOSED FOR SAILING
the second vent is quite small diameter, permanently open ..just enough ID to prevent a vacume from forming when motoring …and this vent is not vented outside the boat…always inside , typically up high in the engine room ventilation system or in a cockpit locker
these boats do not get water in thier fuel tanks
You forgot one source of water... Your diesel supplier.
Round here all the diesel suppliers have their tanks below ground, which is below the water level.
Even my local supermarket petrol and filling station has had trouble with water getting into the tank and has just had them replaced..