: When did you last read STAR MAN'S SON ?

: The major character is descended from the men who landed from the stars - and is mostly human.
: His side-kick is a native to the planet, and is covered by fine gray hair (like a seal), from a species that mostly lives in the water.

What? Sounds like you are describing Starborn, except I don't remember the mermen as being furred. Star Man's Son is set on post-apocalypse Earth.