You can build a wall out but you need to cut back after you are in the water. They won't go backwards much to go around, but it needs to be out in the water.

Also you can dig trenches a certain way so they come in along the shore but get hung up on a ridge... like cutting a channel up and dig it out a bit to catch them along the shore. Again they won't swim out if they get caught in a channel and it directs them closer a bit like a scoop. You can keep them there as pets to slow down other spawns or use them for target practice.

That said though I found it easier to just not build along the shore..