Swimming is great and when you go out in the neck deep part of the pool or lake, do physical excersize standing there. Like sweeping your arms and legs all over the place against the water. It's a terrific isometric work out and you'll actually sweat in the water.
100 years ago I worked at a rehab for amateur and pro sports athletes. It was a favorite part of their workouts. We even had hand and feet fins to add extra pressure.