We have cypress trees at the ponds edge. Many in one pond and a few at another. Theo made good points above. They are a good source of shade and have underwater root systems in the pond (in addition to knees which are modified roots). The point you should consider is that they require pond maintenance. They spread around the pond including to the dam by dropping cypress balls into the water which float until they break open and the seeds contact the pond edge. If they are left in place without being pulled up they will, in time, result in the entire pond edge being covered with them.