@todd A marine or spar polyurethane, when cured, shouldn’t go anywhere—basically any film finish is food safe once it’s cured, in the sense that you can eat off of it, so ought to be ok for the water in the short term at least. I’d find out what people use for their boats. But gradual erosion will happen, no matter what you use, and then there’s plastic in the water. (Of course, there’s already plastic in the water.).

As for linseed oil, use pure and only pure. “Boiled” linseed oil from the hardware store is not boiled but has heavy metal drying agents in it, which by speeding the drying process mimic the effect of heating the oil to partially polymerize it—hence the name. So it’s toxic. I don’t use it for anything anymore. Also, cheap BLO isn’t properly purified before it’s adulterated, and the junk that should have been strained out can make it go moldy in humid conditions.

Oil alone won’t give you much water protection, though. I don’t know what the application is, but you could look into (pure) linseed oil paint, which may give you a longer-lasting surface protection. It is great for houses if applied properly (and if good quality in the first place). Expensive, I’m afraid, but good stuff.