Dock patrolling robot

I was reading the "what do you want your robot to do" thread and I thought
this application deserved a new thread.  I'd like a robot that will patrol a
dock in order to keep geese and swans off it.

It should:
1. Operate at night and charge during the day.
2. Find its own charging station in the morning and start patrolling on its
own at night.
3. Operate between 20 and 90 degrees F.  Operation could encompass deciding
whether conditions are suitable for patrolling.  Maybe the possibility of
ice at low temperatures precludes patrolling.  However, the dock is used
when there is ice or snow on it, so it would be more desirable if patrolling
could occur at low temperatures.
4. Distinguish between the dock and the water (which is sometimes frozen).
The height of the dock is not always greater than the water.
5. Handle bad weather gracefully - if it is raining or snowing, it should be
able to recognize unsuitable conditions and seek shelter at its charging
station until the bad weather abates.  Or if there's snow or ice on the
dock, it could not patrol.
6. Negotiate a fairly steep ramp.

This problem seems pretty straightforward (all the robot has to do is
(slowly) wander around a rectangular area and find its charging station)
until you start thinking about how to handle the details reliably.  What
will happen when rain or snow gets on a camera, sonar or IR sensors?  How to
stay on the dock?

Mitch