I, the landlubber, have heard the terms "the sea is calm" and "a tranquil sea"; but of course they're not specific either.

What about halcyon? Lookit:
Quick definitions (halcyon)


noun: a mythical bird said to breed at the time of the winter solstice in a nest floating on the sea and to have the power of calming the winds and waves

noun: a large kingfisher widely distributed in warmer parts of the Old World

noun: (Greek mythology) a woman who was turned into a kingfisher

adjective: idyllically calm and peaceful; suggesting happy tranquillity (Example: "A halcyon atmosphere")

adjective: marked by peace and prosperity (Example: "The halcyon days of the clipper trade")

From: Onelook