You probably know this, but just in case- you can make pokemon games using the 'Pokemon Starter Kit'.
What you need here is not a terrain tag- if the player stands in the water without surf, it's too late.
Place events along the water's edge. For the graphic, select any sprite with 0 opacity, so it's invisible and stops the player. Make a 2nd page with no graphic. Make it activate when self-switch A is ON. Set the 1st page to 'player touch', and if any actor knows Surf, pull self-switch A, so the event doesn't block the player.