Very useful idea!
In the interim...
mproteau was working on a TLX wilderness kitbash combing Hinterlands ground with the open water from Himmelveil Canals (see ). It hasn't been released yet (kitbashes have been stalled for some time) and the water surface is choppy like a lake rather than linear like a river, but would work ok. (TLX tiles can be used as flat tiles by cutting off the TLX extras and gluing just the core tile straight onto a base board.)
If you're happy with a lower-res solution, Paizo are still selling old WWG sets. Two options here:
1. which provides sandy shore edges and some nice water textures
2. Or the even older with the 'river' tile add-ons from the adventure.
I have a feeling someone made transition tiles to link Skull Cover sand with the grass, but haven't found them to confirm.