I would think you'll want to use a solvent wash to try to get rid of the deck paint.. something nasty and more powerful that acetone (Interlux has several, wear the proper respirators)
As for the hull paint, I'd get a small sample of the paint you intend to use, and apply a small amount (somewhere less visible.. under the counter maybe?) and let it set up to see if the underlying paint is attacked.. if it seems stable and the surface is otherwise smooth then a scuff sand might be all you need. You may still want to consider primer first, but I'd consult the paint supplier about that.
Thanks faster- sounds like a good plan.