I don't think there is much for free out there.

You'd be looking for nautical charts and then look for structure on those charts.

There are a few apps with nautical charts and a few countries, like New Zealand provide them for free on their government websites.

Interesting question...

There are thousands of coral reefs. Maybe 10,000's. No one actually knows how many there are, let alone where they are.

They are usually in tropical waters. But also in waters that are warm both north and south of the tropics. But then again, I have found small patches of coral in cold waters, not big enough to be called a reef, but growing in cold water.