Have a play with DungeonForge and see if that suits you. Its free I think and has splatting, textures and so on. I think it has layers like DJ as well. If your looking to do textured fills like the lake then I would consider trying Gimp as even tho its a raster paint package you can do those sorts of things with it. You will need some textures to help out but theres loads of free texture sites out there too.