Are you sure that the leak is not coming from the Door seal. Those tend to leak and get in the interior......check the headliner do you see water spot?
If you have door seal leak, water gets in the side pillar and make their way down to the footwell and probably get in the HVAC that's why you hear the sloshing water. I doubt it's AC problem from what you are describing.
(Use RTV and run a bead around the rubber seal)