I cannot confess to being a fully qualified electronics expert, just an amateur with a lot of years of tinkering, but there are two reasons why I think it unlikely that the water splash was the prime cause of your problem.
1. A short circuit caused by water bridging two sets of current would more often than not have shown up in the form of some sort of hardware crash while the PC was still on. Probably catastrophic and accompanied by a hiss and smoke emission.
2. Your previous overheating messages. While it was possible to get away without thermal paste/grease with slower and therefore colloer running processors, any processor produced in the last three or four years really should have it between CPU and heatsink.
That doesn't necessarily mean the water may not have contributed, but I doubt it is the sole cause.
This is probably not what you hoped to hear, but I do think you will struggle under an insurance claim.