Olivez,
I am interested in the matter myself, but not so optimistic about EASA-land.
No "DHC6(sea)" appears in the EASA TR list.
http://easa.europa.eu/system/files/d...2025062014.pdf
Perhaps a seaplane class rating (single or multi) combined with a DHC6(land) TR will be accepted, but this is just speculation.
In case you hold a UK-issued EASA licence(?), for a seaplane rating the brits require that you pass also a professional seamanship exam.