Hi there!

When it comes to pen and paper I just did a few maps like that before going on to digital. BUT, there is stuff from my Thunder Rock map that I feel can maybe lift out another technique.
Water preview_v1.jpeg

So, besides the texture of the paper, I literally used pen strokes to signify movement on the water. Plus adding a little highlighting and shading around them to make them pop a bit. Pen strokes like this can add some character to your bodies of water, if you are willing to try it out.

It's simple, but I hope it helps.