What is the best restaurant down by the Riverwalk?

Depends what you are looking for. Authentic Texmex on the Riverwalk, I'd go to Casa Rio. This is real Texmex food that you'd expect to find in any neighborhood in San Antonio. If you don't mind straying a ways from the Riverwalk, Mi Tierra in Market Square is even better. Upscale steakhouse? Try Boudro's. Again, if you don't mind leaving the river than Bohanan's downtown location may be the best steakhouse in San Antonio. For upscale international food, head over to Biga on the Banks.