Here's why:
* Independent clause: "That a very loud boat" (this has a subject, "boat," and a predicate, "is a very loud")
* Dependent clause: "something must be wrong with it" (this has a subject, "something," and a predicate, "must be wrong with it," but it is dependent on the main clause to make sense).
The two clauses are connected by the implied conjunction "so." We could rewrite the sentence to make it more explicit:
"That is a very loud boat, so something must be wrong with it."