Determine if a lat/lon is on water or land.
OnWater API is a free Geocoding API that requires no API key, is served over HTTPS and has unconfirmed CORS support.
Details from onwater.io.

Unified API suite · now live
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The OnWater API is provided by IsItWater (onwater.io).
Yes. The OnWater API is listed in the Geocoding category of the public-apis directory, which only includes APIs that are free or have a free tier. Check its documentation for rate limits and paid plans.
No. The OnWater API requires no authentication, so you can start making requests without signing up.
CORS support for the OnWater API is unknown. Test a request from the browser, and fall back to a server-side call if it is blocked.
Yes. The OnWater API is served over HTTPS, so it's safe to use in production.