I've been to TCI twice and a group of us are going back in November.
Long Bay is fine for foiling but you need to stay south of the main launch/kite school area. Here's a link to a post I made after the first trip. Just below the photos I describe the shallow area north of the launch site. The deeper area is huge. I rode for miles down the beach and back and for a long way out to the reef.
The second time I went to TCI we stayed at the Club Med in Grace Bay again. The wind had a bit more north in it that week and every day I launched right off the beach in front of our room. Plenty deep with some swells to ride out by the reef. We even rode directional surf boards one day, over powered on a 9M. No need to drive over to Long Bay that week.
We did drive over to Long Bay once however. It was a full moon night and a friend of mine joined me at midnight for a magical foil session in the dark. There are no logs to hit like here in BC. The two of us were out riding in the dark and another local foil rider popped up out of the black. The three of had a great session.
Looking forward to going back in November.
If you end up going to TCI I would suggest bringing a wide range of kites. It can be pretty light some days. I was always able to get out on a foil board, but a few times I was on a 13 meter. With a directional or twin tip I think the percentage of days on the water would be reduced.
Ya I was in TCI this March and there were a few foilers in Long Bay, though as said previously there are some shallow spots.
We kited in Grace Bay too one day on northerly winds and that is plenty deep though I imagine the barrier reef gets quite shallow (never made it there), and there are a few other reefs here and there that might be shallow on low tide, but water generally very clear.