no special needed but items to keep in consideration
interior width
overall capacity ( look for something 3500 lbs or more, 3000 is too close for me)
bunks - rails are preferred to add some support to the mid section
and taller bunk standoffs so when entering or exiting the fins don't catch the cross rails
like pic #3 & #9 not # 4
http://www.hewitt-roll-a-dock.com/lifts07/cantilever.htmlcantilevers get easier to crank up the higher they go, and once all the way up transfer most of the lifted weight on to the frame structure and off the lifting cable
vertical lifts maintain the same amount weight on the lifting cable all the way up and are just as hard to crank fro low level to higher level
in a varying water level location verticals are the way to go
there are several to choose from most are very similar
hewitt makes a very good solid lift in both vertical and cantilever, several make good verticals
give me shout if you need more info
good luck
Dan T