Another, more polished, option is http://vegetronix.com. I've used their probes for years and can't believe they're not more popular.

They use capacitive sensing and they're enclosed and waterproof.

I highly recommend them.

$40 is quite steep compared to the rest of components usually used in such a project.

Have you had any fail yet? $40 seems like a lot until you add up replacement of failed units for the cheaper stuff.

I admit the price is steep, but it's worth it for me. They're more for a professional application than the hobby side of things.

I have found them to be reliable and accurate. No failures yet in about a year and a half.

I usually adhere to the old "you get what you pay for" mindset, so I have no problem spending more on something I can rely on.