The quality or state of being aquatic or living in or relating to water.
From 'aquative' (relating to water, from Latin 'aqua') plus the suffix '-ness' (state or quality). This is a relatively rare English formation describing the characteristic of being water-based or water-loving.
Phrenologists in the 1800s actually believed they could detect 'aquativeness' as a specific bump on your skull that showed how much you loved water—turns out they were completely wrong, but they were onto something real about how evolution shapes creatures to love their environments.
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