i launch a 25 foot boat in all kinds of lakes here in So. Cal.; unfortunately i'm only rocking a 2wd, and to be honest there have been some times i wish i had 4wd-low.
i don't know how big the boat is, or how good the ramp is, but as long as you have a steady foot, 4wd-low is more than fine. Just remember you're gearing down big time and in doing so you'll have ass-loads of low-end torque at your disposal...use it wisely. I've seen some pretty pathetic stuff around here lately. (it really makes me wonder)