The source below doesn't indicate that the merganser is a sea goose, but a diving goose (thought it's really a duck--and I have no idea what makes a duck a duck and a goose a gooose):

"1. merganser [n.]
|ETYM| Spanish <mergánsar>, from <mergo> a diver (Latin <mergus>, from <mergere> to
dip, dive)
+ <ánsar> goose, Latin <anser>. , Large crested fish-eating diving duck having a
slender hooked bill with serrated edges; <SYN.> fish duck, sawbill, sheldrake.
2. hooded merganser [n.]
Small North American duck with a high circular crest on the male's head; <SYN.> hooded
sheldrake, Lophodytes cucullatus.
3. American merganser [n.]
Common North American diving duck considered a variety of the European goosander;
<Also called:> Mergus merganser americanus.
4. red-breasted merganser [n.]
Widely distributed merganser of America and Europe; <SYN.> Mergus serrator. "