Depends on your toothpaste. If you're using sonicare + pumice twice a day, then no. Otherwise it's just fine. Be sure and let the brush do the work though, if you find yourself scrubbing while using an electric, you can plan on seeing a periodontist sometime down the road.
The Waterpik should do no harm to the enamel. The enamel is quite a hard susbtance. I believe that Waterpiks actually don't do a whole lot except maybe stimulate the gums a bit.