I have posted a picture below in the related links section of a picture for reference.
Not enough Coolent or theres a crack in the pump.
They actually hold their value pretty well if kept in nice shape. I have seen the older Honda quads go for more than newer Recons or Ranchers.
when your exhaust goes under water and you let it idle it creates a back pressure and the fumes hav no were to got that's why u try to keep the motor at a good rpm under water