The answer to your question would be that "small submarines that can be launched from ships allow explorers to get close to shipwreks and marine life" is the complete subject of the sentence.
The subject of the sentence is the person, place, thing, or idea that is doing or being something. You can identify the subject if you first identify the verb. Then, ask the following question: Who or what(verb)? The answer to that question is the subject. In this case, what allow explorers to get close to shipwrecks and marine life?
