Harvest metals from the dead.
Removal of metal ions using an industrial biomass
with reference to environmental control
It is well documented that microbial biomass is capable of absorbing
metal ions from aqueous solutions even when the cells have been
killed... The biosorption of metals using non-living biomass has
recently been comprehensively reviewed by Modak and Natarajan (1995).
The use of dead biomass eliminates the problem of toxicity and the
economic aspects of nutrient supply and culture maintenance.
Your seas are rich in mineral salts. Water dwellers must exclude these salts from their bodies or they will accumulate to toxic levels - this is exactly the case for fish and other higher animals in Earth's oceans, which must keep the salt from the ocean from entering their bodies in excess.
But once dead, the energy to expel salts is gone, and dissolved materials will accumulate, binding to organic molecules and possibly even crystallizing within dead tissues. The linked article uses that phenomenon to clean wastewater streams.
In your world, dead creatures accumulate metals from the water. The longer they have been dead the more inorganics they have accumulated, until they start to fall apart. Dead things might also stop floating and sink as they get heavier, but a floating dead leviathan is also a good anchor for plants and photosynthesizers. An attached colony of kelp with air bladders could keep the corpse afloat for a long time or even indefinitely. The plants might give off chemicals to suppress decomposers that will destroy their dead float. Also sentient organisms might make constructs to augment the buoyancy of dead things, to facilitate repeated metal retrieval trips.
In your world, floating kelp colonies have one or more skeletons and corpses, ancient or fresh at their center. Your characters will need to get in there to retrieve metal crystals. A fresher corpse might also have scavengers interested in the meat and ready to fight off intruders. Other things might have taken up habitation in such an ecosystem.