I remember 15 years ago people talked of how rivers here in minnesota can harm the hull of plastic or fiberglass canoes. In case your not from MN, our rivers are polluted with chemicals used on farmlands.
Most of us aren't from MN.. I can't imagine how agricultural runoff would eat the hull of any boat in a dilute form. What rivers do you allude to? All of them?
Think I won't ever go to the Boundary Waters again if that were true.
Last I saw on the Wisconsin border people swam in the Mississippi.
I paddled for years in the Annapolis River in Nova Scotia, which ran right through the heart of extensive farming land, and was full of pollution. For many years, people weren't even supposed to engage in any water activities on the river. Likewise, my first wife's parents had a cottage on the upper reaches of the St. John River, in the heart of agricultural land….lots of pesticides and baddies in the water. I never experienced any hull degradation in any of the boats or canoes we used, including, but not limited to aluminium, fiberglass and wood.
UV rays from the sun, on the other hand, will damage many different types of hulls, so keep them stored out of the sun.