The description for this achievement is quite misleading. You don't have to collect all boatman coins, you just have to get to the "Mint III" quest tier.
To complete this quest you have to collect a total of 350 Boatman Coins
I've achieved this at Chapter 14 and collected every coin I could with the gear I've had until then. I did not went back to previous levels with new gear.
I would recommend to search the levels thoroughly until you find the map stones of every map, because with the map at hand every collectible in the level will be marked on your map so you can easily see if you forgot a coin.
as ImaginaryRuins pointed out to me, the next Mint quest-tiers are not visible in the log until you have claimed the rewards from the previous tier. so make sure you collect your rewards in the menu if you want to see how far you are.