The unit is apparently just sealed, with sockets that are also sealed to prevent water from getting into the strip through the socket holes. Water is not a particularly good conductor of electricity (distilled water is an insulator, its the dissolved salt or minerals that make it somewhat conductive) so there's no significant current flow across the contacts due to the surface water. And if it has a ground-fault interrupter, then you are protected from any current that may leak from the socket to you hand when holding it.
Also according to the warning it's not intended for use under water.