We just put up a new website for our automatic chicken watering systems. Check them out! We do sell both cups and nipples for DIY folks too!
http://www.autowaterkit.com

Follow along with the video below to see how to install our site as a web app on your home screen.
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I used aquarium silicone sealant to fix a crack after I dropped a bucket on the nipple. It worked great and it's definitely meant for use under water!...
If you want to plug any gaps for your home made waterer, use plumbers tape. Don't use silicone sealant this is not meant to be in constant contact with water.
I just installed a nipple watering system yesterday and will start using it after I do a bit more water proofing. Curious if the chickens will just figure it out or if there is something we should/could do to encourage them. I don't want to take away their other supply in case it takes them a while to figure it out, but then I also think that if they still have the old one, what will be their motivation to try the nipples. Any input?