Sadly there are no MEP(Sea) courses at an ATO in EASA that I've found. DHC-6 or Dornier Seastars are type ratings not MEP. The problem is the structure you need in licensing behind even one MEP(Sea) course such as FI, FIC, FE or CRE(Sea). The last few MEP(Sea) I saw were N--reg Grumman Goose and Widgeons somewhere in Austria.