I've done this twice with my teen to the Caribbean, once onboard Celebrity Eclipse and another time on MSC Divina. He preferred Celebrity. When we boarded, we found the basketball court which is next to the teen club on deck 15 and within a short time, he had made a few friends. There were fewer than a dozen teens on board and they all seemed to enjoy just hanging out together in the teen lounge, playing basketball, ping pong, etc and video games. This was during March spring break (pre-covid) and before the App was available which will now make it much easier to keep track of each other. The number of teens onboard is usually lower than on other lines but my son preferred that. Probably his worst cruise experience wrt to meeting other kids was in Europe and it was because none of the other teens spoke English. He felt out of place so just stopped going to the teen lounge.