I could not see inside compartments to check where the holes would emerge inside the boat, so I used a pair of small but powerful magnets - one inside and one outside the boat - to show where to mark in the holes. This would prevent drilling into a fillet or a bulkhead, for example.
Here is a cockpit seatback drain hole location marked on a piece of tape, and one of the magnets.
I am nervous about drilling the holes in the footwell. 20mm is a big hole and there is a good chance that the transom could get damaged.