Your shots look good! A tripod is an absolute must. Since your camera only goes down to an ISO 200 there are several things you can do to get a slower shutter speed. Close the aperature to the smallest setting, this will give a nice long depth of field also. Polarizing filters will help also. They generally require 2 stops of extra exposure and will reduce glare off the water. A standard neutral density filter can be used also. Cokin filter holders will even allow you to stack multiple filters.
Something to keep in mind when capturing flowing water... The more flow you have the less time you need to leave the shutter open. If you leave it open too long you will just end up with white going around rocks. In low water situations, like cascades, leaving the shutter open longer can make the falls look more full.