my best friends parents built a 9 hole golf course in 95 sold it 9 years later and it was made into an 18 hole course.
but back when it was theirs my buddy and I would dive the two ponds for golf balls all the time, keep the average ones for our selves and then clean the nice ones and his parents would flip em in the club house.
advice...good pair of goggles, underwater light, a net to hold them in....and take a good shower when you are done.