The Oversea sensors communicate to the Oversea mobile app via a Hub, so they need a Hub to operate. But it doesn't necessarily have to be your Hub! If one of your dock buddies has a Hub and his boat is close by yours (whether in the water or out) then your sensors can connect to his Hub. Some marinas may also offer Hub sharing. We offer sensor-only subscriptions for Oversea users that use someone else's Hub
Keep in mind that without your own Hub, you won't get to utilize its location-on-demand capability or its boat movement detection features. And if your friend with the Hub is on the water, you wouldn't be able to get sensor alerts.