A swedish fiord

Hello!

This is my first post in this forum, and being a complete amateur in
the etymological field but having an etymological interest I am
throwing out a naive question hoping for an answer...

I grew up on the swedish westcoast near the only fiord in Sweden,
Gullmaren. The fiord's name is actually mentioned in the sagas, as
Godmaren, and the explanations has been merely that God- means divine
being and -maren is from latin "Mare" for sea. I think this
explanation is strange, because why would there be a name that is some
kind of cross-breed between old norse and latin? Does anyone have an
alternative explanation?

Per-Allan