The great Duke Kahanamoku spread surfing around the world, He also was an early stand up paddler with knowledge and wisdom of surfboard design.
In this clip Duke stand up paddles on a boat like hull. Displacement hulls were introduced during this era by Tom Blake
Tom Blake stands before the many boards he designed. The boards on the left have round bows or bellies.
Bob Simmons and Matt Kevlin used round bottoms on more modern boards.
Joe Quigg refined the boards and rounded bows remained popular through the 60's
Nose rocker increased, boards continued to have belly.
Belly allowed surfers to recover from a shallow pearl.
Surfboards got shorter and the bows got flatter. Rounded bellies disappeared in the 70's. The theory at the time was, a curled up nose would push water out of the boards path. Similar to the way a toboggan pushes snow. This is true at slower speeds. At higher speeds this happens.
At high speeds, a flat bottom stops from hydrodynamic resistance. The same force that supports a speeding surfboard will stop a speeding surfboard. The flat bent underside protrudes into water, flowing up the wave. A vee or round bottom will part water, the board can continue to move.