You always were a flash bugger, Terry.

Getting back to the main subject, the talk about modes is a red herring - Drunken Sailor can be played on a guitar using any minor key sequence of chords. The problem was as I stated, the whistle player was merely playing Em the relative minor of G. Many tunes especially polkas (Jennie Lind or Bluebell for example) change key between the sections, and many people play sets where the first tune is in one key and the second in another and a whistle player would play both on the same whistle. The two most common keys for folk tunes (I expect because they fit well on the fiddle) are D and G. Em is probably the most common minor key used. Therefore the whistle player was was playing in a key that they were used to playing using a D whistle.

The key of D/Bm has both the F and C sharpened, G/Em has just the F sharpened. Therefore to play in G/Em on a D whistle only requires the player to flatten the C# to a C natural (I think I got that right).