I need to redo the water supply for a new PSS shaft seal. It has 3/8" hose barb. My thru-hull is 1/2" so I will need to use a 1/2" thread to 1/2" bronze hose barb on the thruhull, and reduce it with a barb/barb adapter. That adapter will have to be brass or nylon because they don't make such small fittings in bronze or Marilon for that. (It will become a periodic inspection item, so I'll position it in a very accessible place).

There's also no such thing as underwater rated hose in that size. Cost isn't an issue, it's only a few feet. It will not be under signicant pressure either, as it's only about 2 feet below water, and collapse is not an issue because any small pressure is positive pressure.

I called PSS today and they said that they use double wall fuel hose, so that's what I'm going to use. But I see no reason not to use reinforced PVC hose.