Can constitute be used in the following sentence?
“Every drop constitutes a part of the ocean.”
If not, what word can I replace it with?
I am just guessing here but I think you are trying to say "The ocean is comprised of many drops of water."
It depends on the context of your sentence. As it stands the use of constitute is correct if the context supports it.
For example: "The weather moves water from the land to the sky and back again in a never-ending cycle. Because of this, every drop constitutes a part of the ocean at some point in time."