I don't think there is a real right or wrong. The important thing in the RT practical is to use the correct format for a PAN PAN PAN call. Be concise in the initial call, you can always provide more info later.
Pan-Pan, Pan-Pan, Pan-Pan
Name of station being addressed:
Callsign:
Type of aircraft:
Nature Of Emergency : Overhead a P&O passenger ferry which appears to be in distress
Intentions: Will circle overhead until otherwise advised
Position etc.