Yes, the majority of those boats were reequipped later into mine-sweepers (that was the 2nd class of warships I've mentioned) - and it is interesting fact, not into usual mine-sweepers, but into mine-sweepers with magnetic sweeps/boats for coilless degaussing. They were used for magnetic sea mines clearing.
But the boat on the photo was not reequipped into such mine-sweeper still, so name the small class of warships it belonged to, please. And you win! Varjag mentioned patrol boats and it was true for many tens of ex-fishing boats during WWII, but this case is a little bit differ.
AFAIK those boats were not used in clandestine operations (or I don't have such info at least). As me seems this is not a Lewis gun on the bows, but may be it is as the quality of the photo is not good to understand for sure. But you are very close to the correct answer

Regards, BP