It is a HiPAP device, used for submarine positioning:
https://www.kongsberg.com/globalassets/maritime/km-products/...
https://www.kongsberg.com/globalassets/maritime/km-products/...
https://www.youtube.com/watch?v=MvxeWCoAZPw&feature=youtu.be
That's fairly near the location of Adria-1 submarine cable, so it's possible they are looking at something related to that.
However, they could have just been testing it out in shallower waters near the coast before venturing into 1+ mile depths. Especially considering the mystery piece just recently arrived (according to the DHL tracking).
Per another post that box is part of a fine grained positioning system, so I'd expect the ship deployed it then started doing whatever it was doing, and the fisherman trawled it up in the meantime.