Don't really pay attention to the depths. Sea ducks can be found in any and all waters, from 10 feet to 100 feet of water. You want to find the feeding areas - mussel bars, ledges, reefs, etc. and set up on the X. Do some scouting and find out where the birds are coming in to feed. It could be several miles on on a ledge where mussels and urchins are found, or it could be just off the shore in a mooring field. Eider love to come into the mooring fields to feed on the young mussles that are stuck to the chains. Sometimes, you will find more eider in close than you will far out.
Do some scouting.
Nate