Piscator:
The easiest way would just be to have a large pan, fill it with saltwater and let it dry. I don't know how feasible that is on a rocking boat, but it should work on land without problem.
That's nice and simple idea - as simple as salt. And don't be worry about rocking boat - if we can scramble eggs right now with cooking stone on a deck grill, we will manage with salt extraction
I don't think that salting barrel will be sensible to evaporate salt, because it's purpose is different - we use this to salt some things.
And one thought more - personally I don't think that small amount which you get from extracting salt will encourage a lot of people to do that, so it will be used rather when necessary (ex. salting fishes or meat when you are hungry), not for income, because it's quite a lot of places, you can dock to and dig salt (at least in polish area

)