How can i avoid dehydration during marathon?

Probably you can not: Everyone who runs long distance will perspire to dissipate the body heat made by muscle work, this water is lost and needs to be replaced. Well trained athletes rehydrate less than "amateurs" and have less risk of "exercise induced hyponatremia". I suggest rehydrating along the route with electrolyte solutions if you can.

Training: You must hydrate days before the event. Then keep fluid intake consistent with conditions and with how you trained. You must train drinking fluids throughout your runs. You will find the right combination for your needs. If your urine is not clear or light transluscent then you are at least partially dehydrated. Much written about this -- do some research to help with this. Good luck, have fun.