We use jack lines and tethers when off shore any time someone leaves the cockpit. will even tether in in rough conditions at night in the cockpit itself.
Jack lines should be rigged to reduce your chances of going over the rail. we run ours inside the standing rigging from bow to stern and as tight as possible. Flat webbing is preferred. tethers are ISAF standards, double with a short and long atachment with a quick release at the connection to your harness.
Have a great trip and post pics