Riihimäki did produce water sets / decanter sets. At one stage they were, I believe, the largest glass producer in Finland. The best known is probably Nanny Still's Harlekiini, which included glasses, jugs, decanters etc. there is another waterset and other glasses / decanters illustrated in Modernes Glas aus Finnland (catalogue from a travelling exhibition in Germany, 1989-90).
Their tableware seems to be much less well known than that produced by Iittala and Nuutajärvi.
Interesting to see if there are any other suggestions - so much good quality tableware seems to end up, unfortunately, as unidentified!! :? :?