bag with waterproof section?

I am a total newbie and I would love to make a tote bag with a waterproof section to carry around my and my kid's water bottles.  What type of fabric should I use?  Do you have any advice about which super simple pattern would be good?  TIA!

I would look for oilcloth or maybe even a laminated cotton.
These can be tricky to sew with so I would invest in a
Teflon foot which allows the sticky cloth slide under your pressure foot without getting stuck. These feet are relatively inexpensive and would be worth the investment.
Here is a pattern that features some "wet" pockets: http://annamariahorner.blogspot.com/2009/02/multi-tasking-naturally.html
The pattern doesn't state that there are specifically pockets for holding wet items, but there's a few photos of the same bag with pockets made from laminated cotton. In fact, the easiest would be to find a bag pattern you like and simply use laminated cotton or oilcloth for the pockets.

I bought a thin shower liner and eventually, I'm going to try that. It is like the stuff inside cosmetic cases.....I'm going to go back and check the link posted above.

Carri

I'd love to know if the shower curtain liner works out when you give it a try - clever idea!