Water expands very little with temp, but gases do.

A feverish animal with fart bubbles inside will have higher buoyancy than identical colder animal.

Fart jokes aside, blubber in marine animals changes density with temperature.

https://www.researchgate.net/publication/287491270_Blubber_density_depends_on_lipid_composition_and_temperature_in_three_deep-diving_odontocetes