Sperm whales can go 90 minutes at a time without needing to surface.
I am interested in an idea for a creature that is air-breathing, but spends a lot of time at depths of (100m to 300m) and sometimes briefly on land. An adult is about the size of a human, and is warm-blooded, although not necessarily mammal.
An air-breathing version of a mermaid/man, that spends weeks under water without surfacing.
I'm flexible on shape and form, but I'm after the biological modifications I have to make to such an air-breathing animal who spends its weeks-long migration periods submerged under the ocean surface.