The problems i see here are that if there is more than one actor of the class in the world it get’s hard to identify your actual target actors. As i mentioned i think(and i might be wrong here) that there is only one waterplane for the ocean in the level. The cave and lake waterplanes are child classes and will also get returned but you can rule them out easily by their class.
Physics and post processing volumes are far more frequent in the level and get all actors of class will return a lot. It will be a lot harder to determine the actual physics and pp volumes you want to modify.
It might be possible to store the locations of the volumes you want and then compare them with the results from get all actors of class or use the “get display name” node(i think that this is not recommended but could work in this case) and compare it to the name of the volume in the level.