That's the reason I always rip up those plastic rings before throwing them out. Working for animal control, every year we pick up birds that are tangled up in fishing line, lures, and garbage. The worst is when it's fishing line in a tree or power line and a birds gets caught in it and injures itself trying to get free. With birds like cormorants and herons you have to be careful because they can take your eye out. Those beaks are for piercing fish...they can make quick work of an eyeball. One time we had to wade out and untangle a Great Blue Heron that got it's head stuck through some chain link fencing in the water. That wasn't fun. Not a happy bird at all, lol.
MNR usually doesn't get involved in animal control/rescue. Depending on your municipality you would either call your local animal control, OSPCA branch or wildlife rehab. Unfortunately if the bird is still mobile enough that it can swim then there's not much anyone can do.