You're right, there isn't A LOT of BIG public boat ramps in Destin, but there are a couple that should be able to handle your Boat. BTW, how big is your boat?
There is a public ramp on Hwy 98, right aross the street from the Boat Outlet boat dealer in Ft. Walton Beach, about 10 miles from Destin Harbor. There is another one on Hwy 85, (Eglin Parkway) right next to the Key Bank, in Ft. Walton Beach, which is a smaller ramp. The other nice ramps are on the AF Bases, which aren't open to the general public, which I used when I was there. The problem is there is no parking at these ramps, get there early if you want a parking spot. Maybe someone from FWB or Destin can give more info.
Hopes this helps,