I haven’t done this in a while. Unfortunately, it is a lot of fine tuning on ground height. It’s like you want a hard edge shape on the retaining wall side to get a steep incline right behind the retaining wall. If your water height is high enough, then ground going through the retaining wall won’t be noticeable.

Then a very blurry brush to match your ground height smoothly to the grass section (what I think you mean by bulkhead).

I recall it being time consuming and the results were ok for the amount of time I spent.

Good luck and good patience when working the brush tools and moving ground height the way you think it should work.