I thought that might be the case. Yes water got in but it was dried for 3 weeks before testing. It still went proof...The inner end of that diode lead is ( or was ) really close to the damaged track. If this is a high-voltage part of the circuit, it is just possible that there was an arc there - or, as gaz1 has suggested, some oxidation or corrosion, caused perhaps by water getting in ? has enabled a short.
Are any other components or parts of the PCB damaged or burnt?
With a mains-powered switch-mode power supply, with no circuit diagram, the chances of DIY repair are slim.