From memory, the ill fated Reunion Island site was only 600...

From memory, the ill fated Reunion Island site was only 600 metres from the shore. That would support your theory that there is a steep drop off from these volcanically formed islands. RI is not very far from Mauritius so likely to have similar geology.

I recall from my navy days that Nauru is the tip of a pinnacle thrust up from the sea floor by volcanic action. I remember sailing quite close (about a kilometre) to it in the aircraft carrier I was on, in deep blue water.

https://en.m.wikipedia.org/wiki/Geology_of_Nauru

A drop off of 4300 metres, imagine that? I guess that rules out Nauru for point absorbing wave energy devices.