Water leak in floor board back passenger side

The seals may look fine, but if they have become too compressed and are not making enough contact with the door jamb then they will leak like a breached dam (a lot). My car is doing it too so I have been parking it under the carport instead of the wife's car (she is not happy about that). This happens as cars get old. If you do not want to replace the seal, you can put some Vaseline/petroleum jelly on the seal and massage it in as much as possible. This helps swell the seal up again and it will make better contact with the door jamb, allowing it to seal better.

Also, make sure that the door is lined up very well when closed. You can do this just be feeling the body lines around the door and seeing if they are consistent and flush with the rest of the car.

Try that and let us know.

Also, check and see if the inside of the trunk is getting wet. I have seen some nasty leaks in trunks start seeping into the back seat area of the car. Again, the seals around the trunk would be the suspect here. Good luck.