After everything you have had done to your shoulder... I am shocked you can swim at all!
30 minutes on a bum shoulder is good work!
Are you training for sprints? Or longer distance tri's? If it's just sprints, and you use to be a swimmer (?), then you can probably forgoe too many swim workouts and still do decent in a race.
Best of luck!
"Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather, to skid in broadside thoroughly used-up, totally worn out, and loudly proclaiming: WOW WHAT A RIDE!!!!"