A plane has sensors that should transmit data back somewhere when it hits water correct? Is it possible that these can be turned off/damaged?
Yes, the component in question is called the Underwater Locator Beacon.
They are not foolproof. To disable them manually you need physical access to the black boxes, which are located in the tail section of the plane.
A plane has sensors that should transmit data back somewhere when it hits water correct?
This isn't really correct.
The Cockpit Voice recorder (CVR) and Flight Data Recorder (FDR) don't transmit data. They do have attached to each of them an Underwater Locator Beacon (ULB) but this doesn't transmit data either. When submerged, it sends out a sonar ping every 50 seconds for up to 30 days. This sonar signal can be detected by an immersed sonar sensor in the near vicinity (maybe a couple of miles), not across thousands of miles of open ocean.
Ships/boats typically carry an Emergency Position Indicating Radio Beacon (EPIRB) designed to start transmitting a location signal to satellites when the ship or boat capsizes or sinks.
The aircraft equivalent is the Emergency Locator Beacon (ELT) but, in the USA, from what I've read, this is only mandated for small private aircraft.
Commercial planes that fly scheduled routes are not required to have an ELT (I imagine the regulators thought that ATC communication, transponders and ACARS would together provide adequate location data)
787s do have ELTs fitted, somewhat notoriously. I've seen nothing to suggest that 10-year-old 777s do.
Is it possible that these can be turned off/damaged?
ULBs cannot be turned off in flight - because they are not on. Since the CVR and FDR are at the rear of airliners the ULB is not accessible from the cockpit and probably cannot be easily disabled. A ULB would have to be disassembled (see 02:55 onwards).
The ELT, if carried, is typically fixed to the top of the aircraft and designed to break away if a plane sinks. I imagine this makes it difficult for the crew to access it and disable it in flight.
yes, ELTs can be damaged.