Airboats don't use constant speed props - their props get damaged too
often, CS is much more expensive than wood, and airboats use prop
pitches that are much lower than airplanes because they aren't going
nearly as fast

I've seen an airboat with a CS prop. The internal stops can
be adjusted to get the lower pitches necessary for airboat operation,
and the blades themselves could be twisted a bit to lower their overall
pitch.

Dan