Definitely not a ground plane. My understanding of a ground plane relates to the rf world. Essentially as I understand it a ground plane is a “spring board “ if you will that an rf signal uses to get out in the atmosphere. If you’ve seen a cv antenna on someone’s home or an amateur antenna you may note there are sometimes, depending on the antenna three horizontal elements and one vertical. The horizontal elements make up the ground plane. In a boat that may have a SSB radio, the ground plane would be comprised of a copper screening material that is laid inside the hull and then connected to other “grounds” like tanks, engines really anything metallic to comprise the ground plane. Probably more than you wanted to know!