I'd say a 20' foot...just for the extra space & stability. On a 16 footer you wont comfortably have room for more than one other person.
I have an 18' foot, and in 3'-4' foot wind waves I get very wet (center console) and pushed around quite a bit.
I agree with Bushbear's post. Up in the marine area you mentioned it can go from 'reasonable' to FUBAR instantly.

Make sure it's a V-Hull not a Tri-hull or flat bottom, etc.

Look at it this way: Look for a solid Salt Water boat just small enough you can put it out on a lake, rather than a lake boat just big enough to go out on the salt.