We are looking for anyone who has had experience going from Santa Theresa either by small boat or ferry. We plan on going around 22 June.
We had heard that there are private operators' who will transport you, however, haven't located any yet.
Yes. I've crossed a couple of times form S.Teresa to Bonifacio, with a vehicle too. It's been about 10 years ago though the last time I went.
If you have a vehicle the only way is the ferry which departs from the Ferry dock. The ferry dock is not at the harbor where all the various small boats are. You have to go through the tunnel that takes you to the ferry dock at the mouth of the harbor. The harbor in S.T. is a narrow bay. The ferry stops at the mouth of it. I purchased the ticket on the spot, but you can reserve ahead of time (advised if traveling with a car). I don't remember if I used Mobylines or Saremar. I think I used Saremar, because it was cheaper. Make sure you are there well ahead of time, because I think the check in is at least one hour ahead of departure. If you have no car, then it's probably not necessary, plenty of room on the bridge for passengers only. The cost is probably under €30 for the car (one way) and less than €10 for each passenger (o/w). The crossing takes about 45 min.
The above is for the ferry. If you don't have a car, I'm sure there are a bunch of small boats at the harbor that organize day excursions to Bonifacio. I've used those to visit the Maddalena archipelago but not to Corsica.
Bonifacio is really pretty of top of those cliffs and with that narrow harbor. Spectacular. Don't miss it if you are in Santa Teresa, even if you go for just a day. If you go across for a couple of days, Porto Vecchio is close to Bonifacio and also really pretty.
Make sure you don't look French. They hate the French from the mainland in Corsica. They'll blow up your car with a car bomb if you are French and piss them off. Speak Italian to them, their dialect is a variation of the Tuscan dialect.