I also have to have a window seat on flights due to severe motion sickness, and don't pay extra to pre-select window seat.
Some things that have worked for me:
1) Select a window seat during online check-in for no fee (check-in asap once it's 1 day prior for more seat options)
2) If you can't get a window during online check-in, take the pre-assigned seat and ask for a window seat reassignment at the gate. Especially if you have a doctor's note, but even if not, this is usually no problem (depends on airline)
3) Board last, and simply sit in an open window seat if flight is not full. Seats are often reassigned last minute anyway. If asked, you can either play dumb or tell the flight attendant you need a window seat for health reasons (they definitely don't want anyone getting sick during flight).
Ginger actually makes me nauseous, citrus has helped though, I always take some clementines along. Good luck!