Old Mariner and the amulet

[Sakirth]

Does anyone know what the amulet he's selling is for? And how to even get it? He just refuses to sell it to me because I'm not ready for it yet lol.

Also, does he only spawn appear on Fridays?

[Bexy]

Spoilers:

You need to be in a relationship first. There is a ritual with people in Stardew Valley where the Mermaid Pendant is what you give someone when you want to marry someone. It is the way of telling someone you want to marry. He will sell it to you then

[General Brasch]

Apparently they get you married

[werdry]

At 8 hearts with one of bachelors Pierre will start selling you Bouquets (200g each). This one needed to unlock last 2 hearts and make it 10 (there's also abuse - bouquet doesnt count in gifts limit so you can litterally purchase someone's love gifting ton of bouquets).
Once you 10 hearts, Old Mariner will sell you Mermaid's Pendant for 5000g. Its a gift that also a marriage proposal and make waifu/husband's max hearts to 12.

Its not just for achievement and waifu, tho. Spouce can get 13 hearts (you will see only 12) and when this'll happened, you will get one of 7 stardrops in game, which perm increase your energy.

P.S. Old Mariner spawns while rain and always at same spot, day isnt matter.