Yeah, I knew that we weren't doing much sea travel at the beginning, but perhaps river trade in their own countries? Anyways, if they're too advantageous in the water give them more handicaps on land.
Also I thought that if they are inclined to be in the water but there isn't much water trade or travel, it could be worked into the story, like they are used as slaves on land because they are at such a disadvantage on land so they are easy to keep as slaves and water travel is limited to keep them away from the water.
As for limitations, perhaps underwater for up to ten minutes at a time with the use of their gills (many humans that train their lungs can stay underwater for 3 minutes without a problem). And give them a bigger handicap on land by saying that they are very clumsy on land because they are used to the movements of water, both in and on boats, so they are thrown off-kilter by the stableness of land.
