My boat is already insured. Do I need another insurance for peer to peer boat rental?

In the case of a rental "without skipper", that is without you on board as a skipper, your boat must be covered by insurance for "peer-to-peer rental".

You can get this through your current insurance company.

For any rental "with skipper", that is with you on board in the context of a 'Co-Sailing' (ie shared sailing) arrangement, you need to ask your insurance to cover your Co-Sailing activities. You are in charge and thus responsible for the boat.