Easiest way to measure fermenter temp in swamp cooler?

I only have a few batches under my belt, and up until now, I have not maintained/controlled fermentation temperatures (I know.. I've been bad). Especially with summer coming up, I am going to start using a 'swamp cooler' of sorts.

My main question - what device is easiest to measure the temp. of the fermenter? I've heard about the stickers that measure temp, and are only a few bucks at brew stores. But do these last a really long time, and will they wear off if they end up submerged in the water of my swamp cooler?

I would guess I could just stick a thermometer in the "water bath" of the swamp cooler, and assume the fermenter is like 5 degrees warmer, but I'm guessing the real fermentation temp can vary depending on yeast and other things.

Thanks!

The stickers are ok however don't let them get wet, the ones I used got wet and got destroyed.