In Vermont, it's 200' . I'm luck if I can maintain 100' or even a little less on weekends. My guess is that 75% of boaters couldn't guess what 150' is if they were given a laser rangefinder. Happens on the golf course all the time, and the lake is even harder to guesstimate. I just try to steer away from traffic or go around it best I can to avoid potential problems. Sometimes the tracks on my GPS look like I'm running away from a gang of angry bees
I don't know what the safe distance really is, but I do know many boaters seem to have a different opinion. I prefer to be really far away from oncoming boats, tubers, and especially immovable objects. After 45 years of boating, the only time I would call a close call is at night, when another boat was sitting in the middle of the lake, NO LIGHTS. Amazingly, they did this on purpose. I was already on a focused lookout, because I knew a boat was way up ahead, then I lost track of their lights. Amazingly, they were amazed at my amazement.
