This is a GFCI breaker. It does trip when I manually press the test button, in my bathroom. Why if this is the problem, would the main breaker trip and the receptacle breaker get bypassed? This is definitely puzzling.
Two things here. This is the first mention of a main breaker tripping. Did you mean the GFCI breaker tripping in this post?
Also, if this circuit is protected by a GFCI breaker there is no need for a GFCI receptacle. The receptacle does not provide any extra protection and in fact may not play nice with the GFCI breaker.
Why would the 15 (amp) circuit breaker trip while taking a shower, in only one bathroom? This does not happen while taking a hot shower, in the guest bathroom. ...
Also are you saying that the guest bathroom is on the same GFCI (breaker) circuit as the bathroom you are having issues with? Does that guest bathroom also have a GFCI receptacle?
Your post #16 - at this time I would say that the hot water heater would have anything to do with this issue. Hot water heaters do play a part in other issues such as back feeding a lost leg etc but let's not get into that now.
Have you changed anything in the way of electrical with the devices that are affected where they worked fine prior to doing any electrical work.
Also, you keep mentioning water. Just wondering and may not have anything to do with this but has any plumbing work been done on the house recently. Such as copper piping or copper piping being removed and replaced with PEX.
Another note. Covering the prong holes on a GFCI receptacle is really not such a good idea overall. If moisture is getting into a GFCI receptacle covering the prong holes will cause the GFCI longer to dry out inside (less air flow). Moisture can still penetrate it even with the holes are being covered.
You keep mentioning water