How do you flush water out of the oil system Honda 300 fourtrax?

It will take about 3 oil/filter changes. Put some SeaFoam in the 2nd change. Allow the oil to drain about 10-20 mins each time. and drive it 10 mins between the changes to get the oil hot and worked in. repeat until oil is no longer milky. I would change it again after you first long ride.

good luck.