I agree it would be nice to have a better solution with the introduction of hardened clay. I liked to build brick buildings back in vanilla so I needed a lot of clay. You could stay underwater with a Jack O lantern on your head on the server I played. I literally spent hours on the ocean floor digging clay and crafting it into blocks until my inventory was full. MPS has a breathing module. Mariculture adds scuba. Open blocks sponges could be used. Not sure what mod pack you are using, but if you can find a way to stay underwater, you can have all the clay you want.