I assume you are painting the deck, rather than the topsides(the area between the waterline and the gunwales) since you mention fittings. If you are changing the color at all you should remove the fittings. If you are painting the same color it is still better to remove the fittings. It is possible to paint around the fittings if you're very good and very careful.
If you paint in the water you must be careful to keep the paint out of the water and the water out of the paint. You also have to watch the rigging and the mooring lines to keep them away from the paint.