Open Sea could have procedural temporary content, like islands that spawn when randomly exploring the sea.
Islands could have wreckages, exclusive animals and plants, loot, minerals, resources... The island would be temporary, disappear after you're gone.
To make it an endgame feature it's all about the decay.
You could hop on a rowboat but it wouldn't last 5 min and you'd be left for sharks.
Knarrs could last longer, but to really explore the Open Sea you'd need a big boat like this: viewtopic.php?f=48&t=52724&p=699011#p699011