The issue with maritime insurance, is that ocean going vessels spend a great deal of time in other countries from where the owner registers the ship, and also time in international waters, which are not governed by the laws of any country.

So you average insurance company is not interested in providing insurance.

If you want to read more about the history of martime insurance, you could start with the history of Lloyds of London.

https://en.wikipedia.org/wiki/Lloyd%27s_of_London

Insurance is by contract and either side can take claims to court if the contract is not upheld. I wouldn’t call that a gentleman’s agreement.

The maritime system is complex. But it wasn’t just made up by a few people. It’s evolved continuously, internationally, for generations.