Stand in a lunge position in chest deep water. Hold a kick board between both hands. Arms are bent in front of you with the board completely under water.
Push the board forwards and straighten your arms while pulling your shoulder blades apart as far as possible. Hold this position. Pull the board towards you while squeezing your shoulder blades together. Hold this position.
Keep your abdominal muscles lightly activated and do not allow you body to move. Increase the size of the board and/or speed of motion to increase resistance.