Don't be so hard on yourself! "Nice fire" wasn't the worst thing you could have said, not by a long shot! It may not be the suavest line ever uttered, but it was inoffensive and friendly, so don't be disheartened.
That being said, the most important thing to remember when approaching girls is to be confident. If you are confident, even a simple or lame joke will seem cool. Smile and stand tall. Be casual and relaxed (I know, easier said than done). Just say hello - talk about your camping trip! Ask if she's been down to the lake yet or if she's enjoying her time.
The most important thing to be yourself! You'll want her to like you for you. If she doesn't, it's not meant to be.