It is a requirement for my racing that I have comprehensive insurance with public liability for $5mil, $10mil preferred, and it is checked twice a year. For both my yacht and my racing dinghy.
Also many of the marina's are now requesting to see your insurance policy details when you pull up. I carry a laminated copy to produce for them, as showing them a copy on my phone was not acceptable.
I would prefer to self insure, but my boating uses make that impossible.