They are required to have these tracking devices by NOAA in the USA. The boats have to pay huge fines if they stray outside their allowed zones and are not allowed to fish without out. If you've ever watched "Deadliest Catch", those boats all have one.
These devices regularly report the vessel's position via satellite and have internal batteries and no "off" switch. If you do remove power, the device immediately reports it as a power loss event and you have some explaining to do. If you block the GPS antenna it reports that too and again you have some explaining to do. All events are queued internally in flash so they will eventually be sent. If a vessel is not heard from for awhile NOAA all hell breaks loose since the assumption is that it has sunk, so it's in the vessel operator's interest to leave the damn thing alone.
These devices are quite small, use very little power, and the data throughput is tiny. It boggles my mind that airplanes don't have something equivalent.