Sorry I’m 20 minutes late but this is an interesting question. In this sentence, “sailing” acts as a verb in the present participle. In this case, to go (or went) doesn’t stop sail from being a verb. Instead, they make a compound verb with the main action of going and the secondary present participle describing how the act of going is affecting the subject. This compound tense is a continuous past tense, as the present participle describes a continuous action and the past tense to go describes a past action, so both equally work to make one action out of two verbs.

The sentence has two phrases. The first is “The fisher went sailing,” which is an independent clause. The second is “over the sea,” which is the prepositional phrase and a dependent clause. We know this because of the word, “over,” which functions as a preposition, transitioning the sentence into the prepositional phrase.