If you edge into the wind, like in kiteboarding, with the "edge" of the board, the less rocker you have, the more drag in the water. This is why windsurfers typically go faster than a kitesurfer.
Too many fins cause drag, yes, but not enough promotes drag, as well, by forcing you to dig your board in the water deeper. There's more to shaping boards than bending plywood.
Bonzer bottoms, channels, etc. are all developed to release pressure from the board, as well. The board tends to "stick" to the water from surface tension. If you hold the back of a spoon to running water from a faucet, the water will bend along the spoon. The water pushes on the spoon, and the spoon "pulls" on the water. The wakeboard-style kiteboards require more edge to go upwind due to smaller fins, and you can boost higher, due to the harder edge. Directionals are faster with a round edge but more fins and typically boost highest. Bidirectionals and mutants are kinda the "best of both worlds", but you sacrifice in style and function depending on your riding style. In the end, its all personal preference, but the board's shape affects the ride quite a bit.