I have a 94' S10 blazer with significant rust building up on the bottom of the front passenger door. I guess it has to do something with water drainage or failing door seals. Anyone experience this and could advise on what to look for or any fixes that worked?

Note after a day of heavy rain I hear what sounds like a large pool of water sloshing about on the passenger side...this can’t be good!

 

Drain plugs and weep holes are all over. The floor board ( under the carpet) and drain holes at the bottom of doors. Check that the drains are open on the doors.
I have had vehicles where the previous owner removed drain plugs from the floor and water would work its way inside while driving in the rain.