I'm really into realism, so my views on bodies of water.
Actual moving rivers, going up them shouldn't be possible.
Different types of waters, such as salt, fresh, and brackish.
Different animals and plants based on different water types.
Add rafts and boats, ocean with waves would be great.
For sails, would prefer a system similar to Scum the game, where you go side to side in order to go against the wind rather than paddling like in Valheim.
Boats need to be able to support multiple people.
Need a thirst system where we can't drink brackish or salt water, requiring us to store water in order to travel across oceans.