Idk, I often leave some of mine floating after it reaches the surface and bends to the side. This causes the roots to start growing from the side and down to the bottom. It roots like crazy for me no matter what I do. Getting it to start growing from the side though is not something I have tried.
Mine seems to just split when/where it wants to XD I haven't tried to get it to do that intentionally but this sounds like it could work.
PS - in your pic, which part of the coast are you on? I know it's Ireland but I've been to the far north, east, and west coasts in person and I still cannot tell them apart in pics unless there's a really obvious landmark!