Dan - I don't know about the UK or Croatia
, but in the US one of the effective ways to fix this problem is to be listed as a "Named Insured" on the boat
owner's policy. For liability purposes, if the boat is damaged while you are using it, then you have become an insured under that policy.
Also: Have you inquired of the company who is renting
you the boat? They may have an insurance
You are right to be concerned.