I agree - you are so bouyant in the salt water you just float naturally, and with fins on your feet, you don't need your arms much for swimming or navigating - your legs do most of the work. I usually have my arms at my sides as well, unless I'm taking a photo.

Just be sure the water is calm - the only time I really need my arms is when I've been in a strong current and need them to help navigate (this past July a wave almost pushed me up on top of a reef I was exploring - that would've been a bummer! ouch...)

Snorkeling from shore in calm water should be o.k., just avoid any boat trips since climbing the ladder may be a problem.

Black Rock at the north end of Ka'anapali Baech is great place to go with lots of fish, a great chance of finding turtles, and a fair amount of company in case you need any help.

I had the mis-fortune of breaking my shoulder on Maui a few years back, but still had a great time (other than flinging my food when I tried to eat with my left hand!). Just take it easy and listen to what your body tells you.