Not all hotels that advertise they are "on the River Walk" are actually on the river.  It isn't uncommon to find hotels advertised as "River Walk" hotels that are actually several blocks from it.  How can you tell?  Look at the price of the rooms.  Some hotels are actually closer to other attractions than they are to the River Walk.  For example, if a hotel says it's near the El Mercado (Market Square), it's about a 20 minute walk from the River Walk.  If a hotel says it's near the Alamo Dome, it's about 20 minutes the other way.  Near the Alamo itself?  That could be 3-5 blocks from the River Walk.  None of these are deal-killers - just know what you're getting in terms of location.  On the plus side, hotels that are not on the River Walk may offer free parking.  Plan on spending $10-20/day extra for parking at a River Walk hotel.

 Also, be aware that the river is drained in January for a few days for cleaning.  If a hotel that's right on the river has a great price in January, it might be due to river cleaning week (not all hotels will tell you this).  A great time to visit is when the river is decorated for Christmas.  Also, some hotels on the river are so tall that the upper rooms can't see the river.  If you want a room with a view of the river, ask for a lower floor.  Keep in mind the river itself is anywhere from 1 to 2 flights below street-level. 

The River Walk looks like a horseshoe on its side with the opening on the west and the curve on the east.  Most of the River Walk nightlife, restaurants, bars, and shops are along the curve.  Be aware of this when booking your room - do you want to be close to the nightlife or where it's quieter?  The straight sections heading away from the curve are quieter, especially the southern section.  The Alamo is a few blocks beyond the upper part of the curve.  La Villita is on the river right where the lower part of the curve straightens.  The SA River cuts across the opening on the west of the River Walk and the city has expanded the River Walk north along the SA River, but there aren't many attractions yet on that part and there's nothing on the southern part of the SA River.  An older extension of the River Walk begins at the curve and goes east to the Rivercenter Mall and the convention center.