The Dredge Signet Ring is a reward from a pursuit that you can find quite early in the game, ferrying a long-forgotten island castaway back to civilization. The survivor is so overjoyed he gives you his ring that mentions providing good luck to any captain that holds it.
For those still sailing the islands, you might want to know how to get explosives (opens in new tab), rods to catch Abyssal and Hadal (opens in new tab) fish, or how to complete the Recording Rarities (opens in new tab) pursuit for the travelling merchant. Otherwise, here's how to get the Dredge Signet Ring and what there is to know about its use.
How to get the Signet Ring
The Signet Ring is a reward for saving a castaway and taking him back to the settlement of Little Marrow in your boat. You can find this shipwrecked survivor in the island group to the north-east of The Marrows in-between the first two islands in square L10—he's quite easy to spot because of the SOS written on the beach in rocks nearby. Speak to him, agree to take him back to Little Marrow, and place him in your cargo. Be warned: if your hull is damaged on the way, you can potentially lose him and any chance at getting the Signet Ring, but it shouldn't prove a dangerous crossing. Dock at Little Marrow and he'll give you the ring as a reward.
What is the Signet Ring used for?
There's been all sorts of speculation about the ring and its uses because of its description mentioning that it provides good luck, but as far as I can tell, that's a joke related to the fact that the previous captain wearing it was so unlucky as to be shipwrecked on an abandoned island. What you can do is sell the Signet Ring for $90 in Little Marrow, though if you're holding out faith that someone may find another use for it, you may as well keep it around.
It's not like $90 is a lot of money in Dredge and who knows? Even though it seems like Dredge's main secrets have already been discovered, someone might happen upon a second use for it, even if no one has so far.