"The Pacific Ocean is blue."
Translation:Mae'r Môr Tawel yn las.
Becuase your sentence would be "The blue pacific ocean" whereas the "yn" makes it "The pacific ocean IS blue.
I don't know if you can see the "Tips & Notes" for this section on "Countries". It states: "Although Welsh has a word for ocean, 'cefnfor', all seas and oceans are named using the Welsh for sea, môr • Y Môr Celtaidd - The Celtic/Irish Sea • Y Môr Iwerydd - The Atlantic Ocean"
Whilst this may be in the notes, as a Welsh speaker I would use "cefnfor" for these names, and that is in fact the form seen in the greiriadur gomer i'r ifanc.