You shouldn't be charged Clay credits when using your own API keys. When you connect your own API keys for data enrichment tools, you can use them at no cost in Clay.
For Waterfall specifically, you're only charged credits for the provider that successfully finds data, and you won't be charged for most actions that don't return results.
If you're seeing unexpected charges, please check if you're actually using your personal API key rather than Clay's shared key, as Clay credits are only charged when using Clay's API keys.