Have a 95 saturn sc2. when driving and the car heats up you and smell water in rhe car. The floor boards on the passage side is damp. what can cause this... best case / worst case?

problem solved water in floor in drive floor an behind driver floor rear seat floor water in door drain holes was stopped up in door rain filled door up and over flow thru the panel into the floor of the car check by shake the door and see if u hear water that the problem drain holes is stopped up in the door poor design it also most under the weather stripping on the bottom of the door

under the dash on the driver side next to the door

its caused by your aircon the hose may leaking under your dash board

You can try to snake the drain with a wire or something of that sort. You can also blow some compressed air in the drain to unblock it.

when you say left do you mean looking at the vechicle from the front? if so then left means passenger side and it would be the relay box on that side if you mean the driver side you may be talking about the ABS brake computer box, but you did not say if it had wires connected to it.

You probably could if the threads are the same. Aside from military vehicles I have worked on, the Saturn was the first car I worked on with a spin-on trans filter. Most auto parts stores carry the transmission filter for Saturns and they are not that much more than an oil filter.

either it's undercharged or there is something unhooked/ faulty in the electrical system. more than likely undercharged.

Is it antifreeeze from heater core or heater core hose leak?  Is the leak pure water, or maybe something else?
Put it in the garage again overnight with an old white towel in the drivers foot well, best if you can find a way to stretch it from drivers seat forwar to firewall and keep it off of floor mat.  Check it before you go to bed,  then in the morning if no leak found yet.  see if the color/location of the spot helps you identify the source.  I think the heater core is under the dash center.  Anti freeze is green/yellow or orange.
good luck

first you need to get a flash light go to your pass side look arond the black casing off your heater see if the bottom is wet try with heater on and off usaully when a core is given that much water out when you turn your heater on it will steam your window and you will smell antifreeze if theres any if none of things are present i would check the seal around your windshield around firewall area rain can get into areas that the factory seal might have dried and cracked open leaving you with a wet carpet and a puddle in the back floor pan as for the airbag light you need to have the light reset at a garage could be a stored code in computors memory you can try your self by taking the fuse that says ecm leave out for ten seconds start vehicle see if it reset second way take pos cable off your battery leave off 10 15 seconds reinstall start if light stays out great if not you will need to have reset with scanner like a snapon or rotunda

Open the hood if it says efiohc or single cam on the valve cover then it is a sc1 if it says dohc or twin cam than it is an sc2