I went on a foraging expedition to find the new stuff yesterday, came back with buckets of fish, the contents of three shipwrecks' worth of chests, one hidden treasure, and several stacks of kelp. Once back at base, I decided to set difficulty to easy and go for the Castaway and Sleeping With the Fishes achievements simultaneously. So I made some dried kelp, got me some potions of water breathing, and jumped in a lake, with a clock in hand to keep track of time.

Two in-game hours into the day, after eating exactly two pieces of dried kelp, Castaway popped.

The description says "eat nothing but dried kelp for three in-game days". It popped after only two in-game hours. Not that I'm complaining, because easy achievement, but it's supposed to take longer than that.