Learn Italian: L'acqua del mare è profonda e calma.

In English, you can put two nouns together like 'sea water'. In Italian, you usually express this as 'the water of the sea'. We combine the preposition di (of) with the masculine singular article il (the) to make the combined word del.

Why do both adjectives end in -a (profonda and calma)?

They are describing l'acqua (the water). Because acqua is a feminine singular noun, the adjectives must match its gender and number. The standard dictionary forms profondo and calmo change their ending -o to -a.

Is acqua a feminine noun even though it uses L' instead of la?

Yes! The apostrophe in L'acqua just takes the place of the a in la acqua to make it flow better when speaking. It remains a feminine noun, which is exactly why it requires those feminine adjectives.