What the achievement asks
Loot one hundred Seafarer's Dubloons. Ten points, account-wide.
Seafarer's Dubloons are the island currency, and they are picked up off the ground during an expedition rather than dropped by anything in particular. Small piles sitting in the open, easy to run past without noticing.
There is nothing to do differently
This is a counter, not a hunt. A hundred dubloons accumulates on its own while you run islands for anything else, and there is no way to make them appear faster.
The one habit worth forming is looking down. Dubloons sit on the ground in the open rather than inside chests or on bodies, so they are easy to miss entirely while chasing an elite pack.
Dubloons are also spent, on the traders aboard the ship before each expedition. Spending them does not undo your progress: the achievement counts what you have looted over time, not what you are holding.
It lands while you do the rest
Nobody needs to farm this one on its own. Anyone working towards Expert Expeditioner and its fifty Mythic wins will pass a hundred dubloons well before the end, and the eleven per-island achievements add more runs on top.
If you are picking an order, run islands for the things that need luck, I'm Here for the Pets and Tell Me A Tale, and let the counters finish themselves.