I'm not sure I understand your question exactly. Seawater definitely presents problems for sailors that fresh water does not. For one, you can't drink seawater, which means that someone boating along a river system that stretches for thousands of miles would have very different logistical problems than someone boating across the ocean. Having to take fresh water with you for drinking (and having to be thoughtful about things like catching rain water whenever possible) is a major constraint for ocean voyages.

Another major difference is biofouling (e.g. the growth of marine organisms like tube worms and barnacles on the wetted hull of a vessel). In fresh water this is a fairly minor problem, in salt water biofouling is incredibly aggressive. In a very short amount of time (months) any ordinary boat will accumulate a remarkable micro-ecosystem of critters attached to the underside of the vessel. This not only reduces the speed of the boat through the water by increasing the drag of the boat but it can damage the hull as well, particularly in the case of teredo shipworms. Even in ancient times boat builders have been known to employ various methods of anti-fouling ranging from metal plating to coatings and paints containing a witch's brew of noxious materials. These would be coupled with routine maintenance where a ship is brought out of the water by various methods (e.g. dry dock, crane, careening, etc.), biofouling scraped off the hull and any anti-fouling preparation would be re-applied.

There is even an account of a 10th century Norse boat being lost to ship-worms in the Saga of Erik the Red.

Ancient sailors would have had to contend with biofouling causing a high burden of routine maintenance and repairs as well as shorter boat lifespans. Various anti-fouling techniques (use of pitch or tar, sulfur, arsenic, and other materials) have been in use in the Mediterranean at least going back to the late Bronze Age.