[elli1]

Probable answer ... he was acting captain of the boat, though it may have been moored.

That's a possibility that I didn't think of. All that I've been able to confirm is that the Tattoo sails two nights a week for a 'dinner cruise'. Thus, I'm wondering if the boat operates as a docked nightclub on other nights.

But how is the white shirt relevant?

[maggief]

But how is the white shirt relevant?

It's relevant because, IMO, he's wearing the same striped epaulettes as the captain, therefore acting as captain. That would put him in charge of the operation, giving him access to boats, keys, supplies, etc.