Teen diving enthusiast's project has removed 50,000 golf balls from off CA coast

Yes they degrade over time.

Probably a lot smaller now that cruise ships can't use the ocean as a driving range. Most of the balls from the Monterey Peninsula come from just one golf course, and certainly a lot more than 50000 have entered the ocean from that course alone.

Edit: 50K golf balls is about 3000 square feet of surface area. So imagine three apartments' floor area worth of plastic surface representing that amount.