SR, everyone is different but there is no way I would try and run a marathon with just water. I used to just take 1 gel for a half marathon but my friend (who is an elite runner) had a word and said that I needed to take much more. I took 3 on my last HM and it really helped and I didnt feel as wiped out at the end.
I will be taking gels and have someone in the crowd carrying flat coke for me (runners dont usually use coke, its a triathlon thing and it works for me).
If you think you need gels then start getting used to them. You need to find what and which ones works and which dont. Gels can have a bad effect....I had to stop 7 times during the run at ironman Switzerland for 'comfort breaks'