I have designed a dock in which you have a water stream preventing the boat from going out of my desired perimeter.
What I did was make a wall with a 3 block wide opening facing the water. Then place sign boards on either side so that there is no water stream towards the ocean or my dock.
Here and there. After that all I had to do was put water both sides and the stream facing each other.
At last cover it all up like that.
I did some testing but I would like you guys to test it out too and answer if there are any problems.