How can sufers go forward?

Question

In movie surfs up, I can see sufers go forward without effort. I wonder why it is so.
If necessary, please use equations.

Answer 1

No equations necessary. It's downhill coasting. Surfers are riding a downslope, that of the water on the leading part of a wave. The same effect occurs in whitewater boating, where the river has enough downslope for the canoe or raft to reach a high and scary coasting speed.