If it's tile based game you can always get away with the fact that it's supposed to be like that. It's even possible that blending will confuse players who are bound in tile based strategy (in turn/tile based games). That's why it's better just show the underlying grid clearly.

One of the options is create in-between tiles. Like shallow water and/or sand between water and land.

I think with modern technology it could be viable to create smaller than tile objects like vegetation to draw around the edges of tiles as an extra layer not related to grid itself.