Bulfrog,
Mimosas are trash trees not native to the US. They are short lived, but grow extremly fast. Best situation for a water garden with water lilies, etc is no trees at all, since shade will prevent blooming of the lilies and the annual dump of leaves will be a constant maintenance problem. I'd dump the oak as well or at least build another pond away from the trees. You could grow water hawthorn in a shady pool and it will bloom.