I've worked as a yard hand in a previously life.  Generally, we could expect the vast majority of the boats in the marina to be lifted out at some point.  The majority were mainly sailing yachts who were regular users and need rigging work or hull work doing over the winter months.  I noticed that a lot of power boats stayed in the water and didn't move - often they hadn't moved all summer.

The floor space required for a boat will depend.  Firstly you have to consider the length and width of the boat and you need to allow for any features that protrude from the main part of the boat and whether the yachts cradle itself takes up any space (generally the cradle will fit underneath the boat).  The width will also need to allow for the boat hoisting plant to drive either side to load / unload the vessel.  I would say that to allow for this you'd need to add on maybe 4-6 feet either side of the yachts normal width to account for this.

Marina's are obviously selling space in their yards, so they tend to pack boats in.  However, events at QAB in Plymouth last year may mean that boats are spaced a little further apart - think domino effect!  Best bet is to go down to a marina and measure up or ask the guys down there.  Its just about the right time of year. 

Thanks ASOK, that's very interesting