"I see the sea."
Translation:Я вижу море.
Indeed this sentence does require the accusative case. Морю is actually the dative case of море. You can see all the declensions of this word here https://en.wiktionary.org/wiki/%D0%BC%D0%BE%D1%80%D0%B5#Russian
it is already accusative, but because море is neuter, it doesn't change: http://www.russianforfree.com/grammar-of-russian-nouns-accusative.php