Are there residual devastating toxic sludges in any non-human structures in the ocean

Yes. (Black smokers, white smokers, other discharge points for hydrocarbons .. like tar pits on land, only underwater)

There are also human structures in the ocean that lack toxic sludges.

The volcanic vents are interesting in that, while toxic to most life, separate species have evolved that only live in toxic hot sludge.

There are many types of toxic sludge, the fact that rare organics can live within them not only points to the possibility of life off planet earth, it also hints at potential uses in remediation of human created toxic wastes (binding to heavy metals in wetlands capturing industrial run off, etc.).