How do you measure water?

I am just curious what tricks everyone uses to measure water for cooking.

I used to have a nalgene that was marked but dropped that in favor of a lighter platy.

I use the platy during the day to hold my flavored drinking water so I don't want to have to chug what is left to use it as a measuring cup.

Do you have a good way of marking your pot?
Do you just guess-ta-mate it?
Do you carry a measuring cup?

–scott