why does nami have a nose??????

Because a female fizz would gross people out even more

She's gotta be able to smell your fear above the water too.

to breaf

She's a mermaid and mermaids are humanoid so they would probably be marine mammals and marine mammals breathe air

She doesn't seem to have gills either but its possible that they are on her neck and her neckpiece/necklace thing is covering them

Fish have noses...