Ok, my mistake.Fish can be only in coastal water.
However, limiting WB just to coastal still doesn't solve the problem of them having possibility of exploring whole eurasia + africa for example (until border blocks them). I'm strictly against that (it gives unfair advantage against AI ) and to be honest, I wouldn't mind limiting WB to just staying in own territory. This would be the best representation of WB being fishermen, who sail to certain location from the city every day.