Can I sue the golf course if I fell into a main water line that had no cover and at that time was covered in just mowed grass?

Probably not, unfortunately. First and foremost, whenever anyone plays golf--or indeed takes part in any sport  or recreational activity, that person is deemed to have "assumed the risk" of common dangers that are implicit in the activity. When playing golf, walking all over a course that has water obstacles sand traps, tees, roughts, and no small amount of sprinklers, irrigation, etc., a risk assumed is that of tripping; therefore, tripping or falling into some obstacle on the course may be an assumed risk.

Second, you'd have to show that having the line uncovered was unreasonably careless.

Third, you'd have to show that you were careful--i.e. that you did not fall because you were too careless and did not see the line when the average person would have.

Fourth, in a lawsuit like this, the amount of damages or compensation you can get is linked and related to the extent of injuries you suffered. If you were fortunate enough to not suffer serious injuries or significant medical costs, there is nothing to sue for.