For a while now I've been contemplating a variety of ideas for creating a large body of water on which to play out scenarios with my miniatures. I imagine scattering some islands about and having players' warbands sailing about exploring them.
Check out my blog,
Scrum in Miniature, for a short tutorial on how I got a tabletop ocean, fast and cheap.
https://miniaturescrum.blogspot.com/2018/02/water-terrain-cheap-and-easy.html