I have a character (humanoid) that has the ability to breathe underwater; either through gills or through augmented lungs but I'm not sure what starting list I should be aware of in concerning their water they'd breathe.
I know fresh and salt water is one issue but I'm sure each has their own set of problems but they're both still water (excuse pun there) and would no doubt have a lot in common nonetheless.
I know for us, the air has to be specific in composition and free from toxins but we breathe in contaminants such as pollen and other gasses. I can estimate that water would also contain contaminates of pollution and plants, but given it's much denser than air, it'll carry a lot more and suspended too which would no doubt affect the characters' ability to breathe in something 'clean'
There's also no doubt a ton of marine life; especially on the microscopic level, plants too, chemicals... but I'm not sure on how that'll impact a humanoid's ability to breath or how they can filter some of that out if they're physiology is more...augmented from a human.
So, my question is for my water-breathers, What should I keep in mind specifically before they jump into a random puddle of water and how it would affect them?