Not commenting about all the other replies, but chiming in with another
option, not knowing if you were just curious or were wanting to have
one:

I'm a personal (well, was, as I've sold my home) user of Air-Dock, and
happy enough about them to become a dealer (spam warning: I earn
something for each one I sell; the price is the same to the user - if
you're interested in buying, please let me know).

The principle on which I personally was enthused was that it doesn't
have any hard points. It's basically a pillow for your boat.

Now, if you have a sailboat, the task gets virtually impossible without
some heroics in any air lift, due to the keel. OTOH, nearly any size V
or flat-ish bottom boat can use either the Hydrohoist model (tanks and
trailer bunks, practically speaking) or the Air-Dock.

However, in shallow water situations, the reason I first bought the
Air-Dock, you can use any level water which will float your boat, plus
about 6". Try that with a Hydro-Hoist!

Another bonus is that the end price is usually about half of what the
commercial hard-point floats will cost.

As to raising it, the others are correct except that ball, not gate,
valves, are typical. And, yes, it's basically a vacuum cleaner motor
in a water-resistant housing. Drawing about 600 watts, I've done
installations with an inverter (750W, under 75 at Wallyworld, but I
redo the connections to fit the battery, usually replacing the wire to
allow a more convenient location for the box) connected to the battery.
As little draw as they use in a lift (or many, for that matter), each
time the boat's started, the alternator will bring the boat back up to
full charge and they're ready for the next lift when they come home.

In this case, you have a sealed system - the bag never takes on water
(unless you puncture it - which you could, same as you could puncture a
very heavy duty inflatable dink - but the repair process is way simple,
and each comes with a kit for such eventualities), but instead just
inflates and deflates. Pushing air requires somewhat less effort than
water, so it doesn't take as much horsepower. However, in either case,
the ambient pressure inside, once it's up, is never more than about
3PSI if it's sized correctly.

If you're interested, go to http://www.justpickone.org/skip/gallery/
and click on "airdock" for an installation I did from my boat, as well
as my personal installation. And, if you're interested enough (spam
warning) to pursue it, please do it through me :{))

L8R

Skip, rehabbing, about to enter active PT (currently passive) and
itching to get back to the boat

Morgan 461 #2
SV Flying Pig
http://tinyurl.com/384p2 The vessel as Tehamana, as we bought her

"Twenty years from now you will be more disappointed by the things you
didn't do than by the ones you did do. So throw off the bowlines. Sail
away from the safe harbor. Catch the trade winds in your sails.
Explore.
Dream. Discover." - Mark Twain