Dude, the manga didn't explain much about his power. His only powers are "Creation" and "Deconstruction". If I remember right, you can only use the elements of what the god has blessed you, and since the MC is the god of medicine, he can't use the other elements you've mentioned. and sure he can make oxygen, but he can't use it to propel himself enough to fight the strong current. I have same sentiments as you before but it was explained in the novel more. And his power needs alot of concentration, he can only create and delete an element as long as he has a vivid image of it, the moment he can't concentrate in his magic, it will fail instantly.
tl;dr his power is useful for creating and deleting "elements" but it has limitations.