I have heard of some US riders checking with their agents over kiteboarding coverage by their homeowners policy. Their agents have said that they would NOT be covered for kiteboarding related claims. In other cases the coverage did apply, so I would check with your agent.
In the USA, liability insurance is available through membership in the AKA (http://aka.kite.org
In the UK through BKSA
In Australia through AKSA
I am not aware of coverage in other countries however that does not mean that coverage does not exist.