I have heard such great things about the Riverwalk! While I haven't been there myself, here is a thread you may find helpful. Re: San Antonio visit need suggestions for what hotels to stay in
There are three Marriott properties on/near the Riverwalk.
The Rivercenter and Riverwalk are both located on the Riverwalk and almost if not next door to one another while the Plaza is a two block walk off the Riverwalk which if you're seeking a more quiet and secluded hotel then this would be the one for you. The Riverwalk rooms were renovated in 2014 while the other two were done in 2011-2012.
The Rivercenter is a more striking of a building and probably the one I prefer over the Riverwalk. I'd rate the Plaza also ahead of the Riverwalk, but then I have not been to the Riverwalk since the renovated the hotel.
I'm sure other Insiders will offer some great thoughts on all of these if not more however; these three are the only places I've stayed on/near the Riverwalk.