'cause nothing they know is particularly useful for future generations.

Knowing how to make hydraulic cement isn't useful?

I am quite certain that the 10 000 top engineers and scientists know quite a few things that would be very useful for future generations. Since I am not in that number, and since I am only one person, I do not know what those things would be, but I estimate a high probability that they exist.

But can you make hydraulic cement underwater? I was under the impression that you needed fire to make it.

I'm not sure. The Wikipedia article mentions that the ancient Romans used a mixture of volcanic ash and crushed lime, and you certainly do get underwater volcanoes, so the ash should be available... there are probably industrial processes now, but just mixing volcanic ash with the right sort of mud and getting something that hardens if you leave it for a day or two sounds usable underwater to me.