Best way to remove hull paint?

Sandpaper and elbow grease ?

Depends on what it is.

Some types of paint can be removed with a solvent, but most marine paints can't be.

Try some acetone or MEK on a small area. Do it outside, unless you like big BOOM.

Otherwise, use an orbital sander.

May I ask why you would want to remove the hull paint ? Remember it is not paint but gelcoat, unless it has been painted already. Keep in mind that painting hulls is the last thing you want to do as a very last resort. Pretty much even old gelcoat is restorable--a much better alternative.