The answer is:
Across the bay, fleets of old fishing boats was seen on the horizon.
Subject-verb agreement is the correspondence in number between the subject and the verb. As a result, if the subject is singular, the verb must be singular; and if the subject is plural, the verb must be plural as well.
In the example sentence, there is incorrect subject-verb agreement because the subject is plural (fleets of old fishing boats) and the auxiliary verb is singular (was). The correct version is the following:
Across the bay, fleets of old fishing boats were seen on the horizon.