I keep my 40+ yr old Piver Nugget in a swinging mooring way up Tejo River in Portugal, floating in fresh water all year being, as such, an easy target for the so called "river rot".
I believe that the explanation for this phenomena is easy to decipher: Wood in fresh water rots faster than when in sea water due to the absence of salt, which is known for ages as, for instance, a food preservative which fights the presece of moisture and gives micro-organisms an unfriendly environment to live in.
Out of curiosity, the old sailors of this river, who used to carry arround freights of wine, cork, rice, sand, wood, hay, etc in huge sailing boats used to compete amongst themselves for the rare chance of loading salt
freights, and they never missed a chance of, when sailing down to Lisbon, get some gallons of salt water inside the boat to preserve the wood, generally made out of pine, pitch pine or spruce planking over oak framing. In fact, the boats working in Lisbon (as people carriers from margin to margin) and, consequently in salt water all year, lasted AGES longer than those working mainly further upriver in fresh water.
Here's one of those boats called "Fragatas" (Frigates):
One more thing, river or otherwise rot happens faster in woods that are allowed to go throught repeated cycles of wet and dry. Wood that is permanently submerged takes forever to rot. In fact, boat building wood logs used to be stored underwater because that way, or so the older boat builders claim, they would last longer and retrieve longer lasting planks than if stored outside exposed to rain and sun.