15' and above they'll keep the motor dry with everyone in them. When the "storm of the century" rolled through, at 5 am I had a call that the water was up, then the phone, and electricity went out, at about the time there was a pounding on the wall. I grabbed a flashlight and shined it out the window, to find my Whaler (plug out) on the trailer in front of the house, trying to float into in my window. I quickly grabbed the dog, opened the door to a wall of water and waded out to the boat, climbed in, unstrapped the back straps and let the trailer sink, unsnapped the cable and let the front end of the trailer sink, fired up (as yet hadn't put the plug in), and motored down the street picking up people as my home filled with 4 feet of water (it sat on a 6 foot mound above other homes in the development). I eventually thought of the plug and simply put it in.
I have been on a 21 swamped and other than sluggish behavior, you wouldn't know it was swamped.
I wouldn't sell a Whaler short, took a long time to part with my two big ones even after I'd bought the C-Dory (and I still own a 17 Montauk).