I use one similar to one at floatswitches.net. I have it wired into a extension cord. When I bought it on ebay, it was said to be safe doing this as long as the pump met the specs of the switch.
I would be interested to know more about the relay set up though. This would be safer. I have used relays before but on different appilcations. Are you using a smaller power source with the relay?
*180 gal Display, 100 gal basement sump, 33 gal refugium, 3x250 MH, 2x160 VHO actinics, zoos, some softies, LPS & lots of acros and other SPS.
*100 gal prop tank plumbed into main system w/ 2x96 PC lights and 1x150 MH,