Most likely a bad heater. If you see a 3"X 12 to 13" stainless steel tube thats your heater, it has 2 leads or wires coming off it, you can unhook those 2 leads and let them hang free as long as they dont touch together or any thing else. Then turn the power back on and see if the breaker still trips, if it does it probably is something else, if it doesnt trip and stays on its probably the heater element? If the breaker doesnt trip, shut it back off and re-hook the heater back up, then hit power again if it trips its most likely the heater.
You still could have other issues causing the breaker to trip.
Also make sure any slice valves are open if there are even any on that spa?? Should be 2 valves on either side of the pumps or heater??
If all else fails or your not comfortable doing what I suggested call a tech!!!