Sealing leaks from outside never work for very long. Any deck fitting that leaks needs to be removed, cleaned, and rebedded.
Unless you can actually see the leak from the stanchions or toe rail, I would not assume the leaks are from those areas. Leaks often show up from far away from the actual leak. Looking at the photo, I'd first suspect the port light above the electrical panel. Knowing under what conditions the leak occurs can be a valuable clue.