Water exercise and land based exercise are both great for breast cancer survivors.

A study published in 2013 divided breast cancer survivors into land-based exercise, water-based exercise and usual care for an 8 week intervention. They found that land-based exercise resulted in a greater reduction in fat mass and increases in lean muscle mass, whereas water-based exercises resulted in greater improvements in breast symptoms such as pain, swelling, range of movement and sensitivity.

Fernandez-Lao, C.  2013

https://www.ncbi.nlm.nih.gov/pubmed/22864470