If a board is the best you've got, wetting it down is worth a shot (or any flat material). It is likely to have the advantage of a lot of area to make up for the lower reflectance.

I'd make sure it is the best I had, though. Flat window glass is better, and flat sheet metal better yet. There have been several rescues due to reflected light from ration tins.

The wet board suffers from:

[1] lower reflectivity
[2] poor flatness
[3] the problem of aiming

In detail:

[1] lower reflectivity

You're basically reflecting off the surface of water, which is less reflective than a mirror, though the disadvantage does drop at shallow angles[A]. A good glass mirror has a reflectivity of better than 85% When signaling directly to the sun (e.g., East at dawn) the glass mirror is 40 times more reflective than water, and plain window glass twice as much. When the sun is directly overhead (noon at the equator) and you are signaling to the horizon (90 deg sun-mirror-target), the glass mirror is still 29 times as reflective as the water, and the window glass 1.8 times as much. When the sun-mirror-target angle is 135 degrees, the mirror is still 7 times as reflective as the board.

[2] poor flatness

What makes a good signal mirror surprisingly bright is that the sun beam reflected off a flat surface is very narrow and concentrated - about 0.54 deg in diameter, or the apparent size of Lincoln's head on a penny held at arms length (or a full moon). If non-flatness doubles the width of that cone, the brightness drops 4x (though it is admittedly easier to hit something). That's why, as an improvised mirror, the window glass is far better than the wet board - the flatness is better as well as the reflectivity.

[3] The problem of aiming

What makes a real signal mirror much better than a plain mirror is the aiming aid. As the maker of this week's video learned, it is far easier to hit your target with a retroreflective aiming aid than with the "vee-finger" approach.

[A] See Fresnel's laws for reflection here:
http://en.wikipedia.org/wiki/Fresnel_equations