Aquatic walking classes can help soothe pain and keep you in shape

http://www.arthritistoday.org/fitness/walking/types-of-walking/water-walking-benefits.php

Arm Lymphedema Exercises

http://breastcancer.about.com/od/lifeaftertreatment/ss/arm-lymphedema-exercises.htm

Page gives exercises, with pictures. These are land but could be adapted to water.