Hi,
these are known as Carolina Bays; don't know how deep you've been searching on this; they were studied at first in the 30s, and at that time dozen of theories came across to explain them, without success.

However, what we can tell about them:

* they're all aligned NW / SE;
* they size from 50m to 10km;
* they're around 15m deep at max;
* most of them have a sandy edge from 0 to 7m, the highest edge being SE;
* they frequently overlap each others whilst it does not affect their shape, one or many small ones could be in a big one;
* the ground beneath show no distorsion;
* they only show in non-solidified sediments;

In fact, the best theory that fits so far is that it would have been created by a comet impact: fragmented in the atmosphere, the C-Bays would be the result of the shock waves of the fragments exploding close to the surface; the "point of impact" being between Hamilton & Puerto Rico.

It's the best shot so far, no tsunami for sure.