I'm a tour guide and I've never heard of surf board rentals here in Munich. Perhaps the reason for this is that surfing the artificial wave in the
Isar is a bit dangerous and not suitable for beginners. I.e. under the wave are loads of rocks. If you don't know what you are doing, you could get hurt! Also, I've seen guys break their boards after slamming into the side wall.
If you do happen to find a place, please post it! My customers often ask about the possibility of renting surfboards!