How could we dump it to the bottom without surface-dwelling scavengers tearing through the carcass?

Torpedo them down there

Besides, when does anything that we try to do to improve the nature, actually help it? Besides limiting our access to it, of course.

From what I've been able to gather, scuttling ships to provide artificial reefs works: it increases the surface area of part of the ocean, which in turn leads to an increase in bio-density and species variety.

Note that this is a vague impression of a topic I know next to nothing about; I could easily be wrong here.

That's somewhat fatalistic and cynical, but there's a kernel of truth in your sentiment. We have of course done good and meaningful work when it comes to environmental protections and preservation - sure, we could argue that perhaps we haven't done enough and should do more, but the effort is there.

I'm not sure if our primary goal in "improving nature" is to help "nature" or simply keep nature sustainable for humans and future humans.

good point, yes. what about the loads of hormones and antibiotics pumped into these? would we want those also to enter the ocean floor biology ?