I just bought one because it seemed like a bargain - don't know why though as I don't use the bigger 5W one I bought last year.
Far better would be a Forgen wind turbine on the A frame. That is the vertical type which is less efficient but far more usable than the normal kind. The advantage of wind power is that when you need the current for pumping water from your bilges the rain is usually accompanied by plenty of wind - especially at night.
Of course the only time a solar panel is of any use is in hot weather when you don't really need all the power!!!
Trouble is the Forgen is over £300 - anyone know of any used ones???