It's complicated. Many ancient civilizations were polytheistic. Old seagods became creatures of later folklore. The gods were largely forgotten but parts of their stories lived on. If you go into a Starbucks for coffee, the woman depicted in their cups is Melusine, an ancient sea goddess with two fish tails.
The earliest boats were human, and later, wind powered. Charts were inaccurate, or just as likely, unavailable. Early sailors were generally illiterate, there was no technology, so folk tales were told to pass the time. The sea is inherently dangerous, so a watch had to be kept. Eyes tired from looking at nothing but the sea and sky, and a vitamin-poor diet, could cause the flash of a surfacing fish, or even a rolling wave, to appear to be a woman to a man who hadn't seen a real one in months. The human mind likes to see familiar things in indistinct shapes.
And that danger I mentioned, and the shops lost to 'bad luck', led to those sailors becoming fairly superstitious. They felt there were things you must and mustn't do to have a successful voyage. So yes, there's a certain element of appeasement. Some of that transferred to the population that stayed on shore, and let's face it, most people like a good excuse to party.
Sorry for the wordy answer. I like to read a lot when I have free time.