Ouch.

Your PT really should have some suggestions for you on what, if anything, you can do for cardio.

If you can walk 30 minutes (! that's a long walk with an injury!) I don't know why you couldn't swim with a flotation board, flutter kicking your legs only until your shoulder recovers. Depending on where exactly your shoulder injury is, possibly you could do whatever that stroke is they used to teach in lifesaving, with breast stroke arms and flutter kicking your legs? Or a dog paddle, even?

I'm remembering from swimming after my ACL tear years and years ago, that I had a lot of trouble with foot cramps because I wasn't fully extending my knee. If you can walk on the leg, I'm assuming you can extend it to within 5 degrees, but you just have to remember to keep it extended while doing a flutter kick.

I wouldn't do unilateral resistance exercises though. Muscle imbalances are worse and harder to remedy than a little temporary atrophy, IMHO.

Sending healing vibes your way.