Learn Polish: Morze jest niebieskie.

In Polish, adjectives must match the gender of the noun they describe. The dictionary form for "blue" is the masculine niebieski. Because morze (sea) is a neuter noun, the adjective changes its ending to -e to match it, becoming niebieskie.

How do I know that morze is a neuter noun?

You can usually guess a Polish noun's gender by its last letter. Most neuter nouns end in -o, -e, -ę, or -um. Since morze ends in -e, it follows the standard rule for neuter words.

Does niebieskie cover all shades of blue?

Niebieskie refers to a standard, lighter, or sky blue. If the sea were a very dark, navy blue, Polish uses a completely different color word: granatowe.

Could I just say "Morze niebieskie" and skip the word for "is"?

No, you need the verb jest (is) here. While some languages let you drop the verb when describing things with an adjective, Polish requires it in this type of sentence, just like English!