Symbiotic relationships are an example of animals using each other as tools for survival.
Examples of simple tools used by animals are sponging dolphins. Bubble nests. Wrasses and rocks. Water in a sense is also a tool. Fish fanning eggs with oxygenated water. Archerfish spitting water at prey.
Complex tools require more of an art that's not demanded among aquatic animals. But if you were referencing to humans going aquatic. Then also yes. We could manipulate other animals as tools for survival. If we lived in a aquatic vehicle, we could build tools on that. If we had floating islands, we can just build off of that. If we became mermaids we could make weapons off of dead animals. We could create net traps to catch prey. Obviously, the possibilities are only limited to the designer's imagination, in truth they're limitless.
I feel that they could create tools by using natural hydro thermal vents could be used to heat up the metal they are using and then shape. Of course to cool it down all they have to do is pull it out of the heat vent into the water. Possibly this would cause them to build large cities around the heat vents, and they would likely be extremely aggressive to anyone who they saw as trying to steal the vent from them.