There is actually an international standard for this: The Chicago Convention on International Civil Aviation, and in there Annex 12 – Search and Rescue, appendix 2 "Ground-air visual signal code". Quoting it:
2.1. Ground-air visual signal code for use by survivors
2.3. Symbols shall be at least 2.5 metres (8 feet) long and shall be made as conspicuous as possible.
Note 1.— Symbols may be formed by any means such as: strips of fabric, parachute material, pieces of wood, stones or such like material; marking the surface by
ramping, or staining with oil.
Note 2.— Attention to the above signals may be attracted by other means such as radio, flares, smoke and reflected light.
So it would be the "V" for the case of the castaway, or "X" as the more urgent distress signal if there is indeed a need for medical assistance. And if (s)he sees the aircraft rocking its wings (or at night, flashing its landing lights twice), that's a good sign … it means the pilot saw and understood. (Specified in the same Annex 12 document as linked above.)