I was thinking what would make life on the high seas (and rivers) more viable and came up with two essential things:
1. Adding a cabin to the Knarr and Snekkja which would function like a small indoors, used for crafting, eating etc.
2. Being able to load animals onboard.
This could enable some really neat gameplay like piracy or make nomads' life easier. Will also help in moving around the world and trading.
EDIT:
Just realized Knarrs and Snekkjas didn't have cabins, so that won't work. Shame