Since MHG sweimen has no descendant in standard German (it may survive in a different branch of High German), the comparative method is helpful. Cleasby, Pokorny, Kluge et al are all a bit dated and don't always agree, but there's not much else available for High German. Much of the OHG corpus is a different dialect to the precursor of standard German or the MHG corpus anyway, so it can be a bit of a strain finding a cognate Middle or Old High German word that resembles a modern word semantically, and surnames may come from a dialect that isn't well represented in a German-English or even a thorough German dictionary.
Schwemm with double mm should be an ablaut grade of schwimmen, probably with umlaut (according to Kluge it's the factitive, i.e. causative — OE has swemman, Danish svømme), but with surnames that can't be relied on, and I don't know how the vowel would vary with different dialects (and some dialects seem to use a different orthography than standard German for the same phonemes).
This message was edited by the author on July 22, 2018 at 6:39 AM