Thanks to all who answered so quickly. I know it was an odd question. John, is there anything I can follow up as to which units in the PDC area employed such vessels or is that like asking which Flotilla employed boats? I realise this is a 'above-and-beyond' type question, so any help is genuinely appreciated.

The boat in question was employed during the rescue of a Bf 110 crew that ditched in the Channel during the BoB and was subject to a well known sequence of photos that also involved a He 59, so people are correct about what this boat may have been doing.