IMHO, you probably have a leak, check the expansion tank as these typically fail after 5 to 7 years. If this has failed, the potential overpressure during the heating cycle can force water out the thermal over pressure valve. I also suspect the tray at the bottom has rusted or otherwise developed holes over the years.
You will probably need to have the hot water heater drained and removed, and the entire wooden structure inspected to see if or how much damage there is and what needs to be replaced or rebuilt. Do not skimp on this a this is supporting the water heater. Once all of the repairs are made, I am guessing you may need a new water heater also, some drywall, and a water sensor in the new tray.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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