One thing that you can do if you have the NAM pedmall tiles installed is simply flip between sandstone, cobblestone, etc etc.. and find the texture you want. Place some on land. Now as weird as it seems, you can place them underwater with ease. Just like you do on land. So place some on where beach meets water, and keep placing them underwater, going deeper and deeper until you cannot see them, and you have seawalls.

This is also good for underwater cars.

haha nice I learned something new today

Thanks. I also prepared a Q&A.

Q: Can ferries, boats, and other water vehicles pass over the pedmall tiles in UDI or else?
A: No, it does not seem to effect water automata

Q: Can bridges be built over these?
A: I doubt, but they might be able to. I will check later.