I am really sorry for my late reply.
Are you sure the water is coming from the floor? if yes then you can buy the 60rupees grommet packet (grommet -rubber covers for the holes in the floor).
Please cross check if the leak is from antenna mounting point? or front windshield gasket/beeding etc. If it is windshield/antenna point just apply Anabond 555 (transparent glue) to outer side, it will solve the issue. I will give you those pics by evening.
To fix the rusting issue inside, just do as Shantanu said but dont use handbrush, finishing wont be good. Instead take time, gather funds then do like this.
If you have no holes/cracks in the floor then
1. Take a screw driver/wirebrush rub the floor where ever you find rust
2. wipe off the area of all the dust using thinner dipped cloth
3. But any red oxide can (60Rs) & mix it with petrol/thinner (1:1ratio) and apply using spray gun
4. let this dry apply one coat of any paint with brush or spray gun. & apply repeat coats later.
500Rs. will be material cost + compressor rent (may be max700) but worth it. OT
: do not buy any spray can, it is a useless product (just looks only) when compared to handbrush and compressor aided spraying.
Yes you can put a simple foam sheet below seat to avoid this water contact. Picture is there in my ownership thread.