Since I asked about these before my trip, I wanted to report back. We were in Tobago for a day recently and managed to find a "route taxi" to take us to Store Bay. It was 8TT per person, but we gave him $10 US for the 4 of us. At Store Bay we arranged for a glass bottom boat tour. Unfortunately, the boat actually left from Pigeon Point, not Store Bay, so after the tour we were stuck taking a regular taxi back to Scarborough. We were quoted $50 by a couple of drivers but managed to find one for $25.
The "Route Taxi" was lined up on the side of the street that runs in front of the cruise port but outside the port fence. We just asked until we found one.