1998 Renault Megane water on floor of car

one poss is if you have a/c the drain is on the pass side front chech the rubber pipe is connected correctly
2nd if you have a sun roof check the drains are clear air line will clear
3rd has the side door trim been removed at any time as behind it there should be a plastic sheet
and if there are any holes in it it will leak into the footwell.

hello there sounds to me like its the heater matrix!
have you checked your coolant level?
if it has gone low recently this could mean that it is leaking onto ure passenger floor as tis is where the heater matrix is located.

Have had the same problem myself, I had a look at the sunroof and noticed that the seals were letting the water through and it was gathering towards the front, and when it reached a high level will over flow into the interior of the roof and down the roof support at the frfont of the door into the foot well, I've siliconed my roof seal up for the winter.