Check your charts. There should be letters like m for mud or cl for clay or r for rock. There are places in the North Channel that are all rock with fissures. You hook the fissure and you will have a hard time pulling up. A trip line rigged to the crown of the anchor will help in pulling the anchor if it gets stuck. There are plenty of mud bottom anchorages all around the N Channel.
I think the harbormaster is trying to scare you into docking vs anchoring. The 5 lb bucket is for small aluminum fishing boats.
There are no protected spots around Otter islands. Have you considered the very protected cove on the SW side of Cove Island?
Be careful of the submerged pilings in Rattlesnake harbor.
Try Club Island, good harbor.
You should get the Ports Guidebook