Also, for those LA's in the northeast, how do you manage the winters?
Prepare for a higher heating bill than you imagine.
Do you shrinkwrap it with a door?
Some people do, and it creates a nice greenhouse effect underneath. You can do clear shrinkwrap and not feel like you're as much in a cocoon. A cockpit enclosure can be used the same way. On sunny milder winter days it's possible to sit comfortably in an enclosed cockpit.
Do your marinas use a bubbler system?
Most do, or will let the slipholder. However, it's really more for the protection of the docks than the boats.
How about pumoput service? Is there a pumpout hose long enough to reach your slip?
If an area has enough of a critical mass of liveaboards a pumpout boat may run all winter or a marina may leave a self-serve pumpout station open. It depends on the marina and area. Either way you'll probably gravitate to the shoreside facilities more in the winter.
Do you have to insulate the fresh water hose?
Most marinas turn the dock water off. Sometimes if you're close enough to the landside you can run a submerged hose where being underwater insulates it. Some marinas also may leave a self-serve water filling station available otherwise you have to bring your water in jerry jugs.
What do you use for heat?
Electric, diesel, and propane are the most popular choices or some combination thereof. Each has its ups and downs which have been discussed at length here on other threads. As I mentioned above, heating an uninsulated boat is much more expensive than you'd think and condensation can become a real issue.
What about clearing the dock of snow? Does the marina do that?
Depends on the marina but generally yes.
Is living onboard in the winter a real challenge?
More challenging than living ashore but nothing you can't overcome if you know what to expect and are willing to accept the downsides.