You don't contract yn when it means "in".
Wyt ti'n canu? (Are you singing?)
Wyt ti yn y bath? (Are you in the bath?)
Yes, "cefnfor" means "ocean" but "môr" can mean "sea" or "ocean", especially if you're using American English or in phrases like this (Y Môr Tawel).
"if you're using American English"
After all, America has no seas, only oceans.
Maybe that's why US English uses the word "ocean" more than we do in UK English then.