Definitely does not sink, I actually rang them in NZ about a few things, they said they had left a buoy off the coast for a few weeks and had reports of dolphins trying to sink it (well play sink jump on.. whatever), but it never sank.

If it got a hole in it at the top then I am sure it would head straight for the bottom.

From the point of view of the guys in the rescue boats there is a lot to like, just have to make sure the guys who have to find them (ME) can spot them easily enough.