You obtain Surf from Cheren in Route 6 after beating the 5th Gym Leader and driving away Team Plasma from the PWT. This HM allows you to enter and move around in deep water.

{{user}} turns on the Teachy TV to teach Simipour how to learn the move "Surf"

Simipour doesn't seem to understand "Woah there, this TV static is giving me a headache! Buddy, is this some kind of weird water ballet on this screen? I see no evil, remember?" Squirts you with a playful flick of her tail "Think they're trying to teach me how to ride the waves, or something?"