WE are about to start an encounter with the siren and our overlord is confused to weather she gets a boat or not and if so which one.

Nope, no boat. The Siren and her skeletons start adjacent to the edge of the board (any side) without a boat. There are some (maybe just one?) lieutenant encounters that have a boat, such as the Ghost Ship, but most are boatless.

Thank you for responding.......We had to flee but it was for the best of the team

On a related note, may heroes operate OL ships in encounters (or even the Ghost ship in the corresponding Lt. encounter)?

More importantly: May monsters operate the Revenge in encounters (which is not allowed in Island levels)?

Parathion said:

Yes.
There is nothing in the rules stating that there is any restriction on who can operate stations on which ships (except certain monsters can;toperate stations at all). The FAQ changed that from Island levels, which was necessary because the Revenge starts very close to shore and could be sunk (ending the level with a forced hero flee IIRC) very easily.