Volcano
Sam learned about the behavior and characteristics of these sea urchins in every possible way he could think of.
Now, he has to make something that could effectively defend against these things and repel them away if possible. And he shouldn't forget about the harpoon guns and the flat-fish puppets.
The harpoons they shoot are also going to be a problem.
But he is a bit relieved by the fact that the harpoon and the sea urchin are a bit similar in their attacking method.
The only main differences are size and shape and the spikes of the sea urchin cannot be defended with water elemental attacks and the last difference being the trap and cable mechanism of the harpoon gun.
The propelling principle is almost the same. But instead of a compressed air contained that is used in the harpoon gun, the sea urchins are creating compressed gas propulsion by themselves.