Jellies in Boston

I saw three jellies as I was walking across the bridge for the Freedom Trail towards the USS Constitution Museum (in the North End). I was on the North Washington Street bridge so I couldn't get a great look at them, but they were about dinner plate- sized. The ones I saw were very near the bridge by the small docks attached to it and were not near to the rocky shore. There wasn't much trash which is great if I recall correctly.