RE: http://www.military.com/military-fitness/navy-workouts/swimming-without-water
I played water polo and the only thing that keeps you inshape for swimming off season is lot's of cardio. Stay off heavy lifting. Med weights, high reps. Lot's of cardio. Most FOB's have halfway decent gyms, use lat pulldown high rep. Just becareful and make sure you don't over do it because it's easy to tear cartilage in your rotator cuff.

thanks!