If your floor board is wet chances are the heater core is leaking. If the heater core is leaking, it would most likely be the reason your coolant is boiling.
The coolant boiling is just a byproduct of a leak, it's not actually the issue. When a leak forms in the system, the pressure drops, lowering the boiling point of the coolant within. As a result of a pressure drop in the system, the coolant's boiling point falls back to normal and the engine heat begins to boil the coolant. This is the reason we use pressurized coolant systems in the first place, to increase the boiling point of the coolant locking it into a liquid state at a much higher than normal temperature. Simply, if your coolant is boiling it is probably because there is a leak in the system somewhere.
You are on the right track with pressure testing the system. Just find the leak, patch it up and you'll be good to go. I really hope it's not the heater core, though. Pulling the dash will be a real pain.