The shoreline would be your reference point.
A reference point, in physics, is some point in space used to illustrate the location or position of other things, relative to this reference point. So in this case, the shoreline is considered as the reference point to locate the position of the surfers.
A reference point, in physics, is some point in space used to illustrate the location or position of other things, relative to this reference point. So in this case, the shoreline is considered as the reference point to locate the position of the surfers.