Tony's suggestions are right on IMO. At ISO 400 the Rebel XTi is going to allow some digital noise which is probably degrading sharpness. At f7.1 your depth of field may simply be too narrow to get the whole frame front to back in focus - f16 or so should help with that issue.

Additionally, it appears to me that your camera auto-focused on the water rather than the rocks and trees. That can happen as the lens and camera search for an area of contrast to help it focus. It may have picked up the light adjacent to dark areas of reflection in the water and locked onto that (I assume the XTi in its default mode uses multiple points like 9 or more within the frame at varying distances from the center and whichever one locks on contrast first determines where the point of greatest focusing accuracy is - like down in the water maybe). So, in effect your camera front-focused, leaving the most important part of the image relatively out of focus.

You can certainly use auto focus but hopefully your lens allows you to tweak focus manually after it has already auto-focused. Most situations lacking enough of the right kind of contrast cause me to turn off auto-focus on the lens and just focus manually.

This looks like a lovely spot to watch for light, season and weather for some really nice images!

PS: Be sure and turn OFF the IS on your lens when using a tripod. If you're going to use a tripod (I certainly would for this kind of image) you might as well use your mirror lock-up option to reduce camera vibration. Along with mirror lock-up it's best to use the shuter release delay timer to provide a 2+ second delay between pressing the shutter button and when the aperture actually opens. That allows vibration in the tripod to settle before the image is captured. That in combo with mirror lock-up are critical for getting the sharpest possible images.