People seem to think there is only one type of '540' motor, looking at the above posts. If you had one of the high amp 'Tamiya' type RS540 motors fitted in your tug, you could expect a total run time of no more than 15 minutes, followed by everything in the model being mighty hot. You state a run time of an hour before everything stops, I think you have a lightly wound 'low drain' 540 type motor. This would provide a run time of between one and two hours depending on the prop size. I think, as stated above, that your battery has dropped below a level that has resulted in the speed controllers BEC tripping out. When you then take the model home, the battery recovers just enough to start working again. Remember that you are now operating out of the water and the current drain will be a lot less than it was in the water. You will probably find that after about ten minutes of running, the BEC would trip out again as your voltage dropped.
For the record, I have not come across a decent 'low drain' 545 motor yet, despite much searching. Unless anyone knows otherwise?