Japan's largest freshwater lake, 琵琶湖 Biwa-ko 'biwa lake', got its current name from its elongated shape, which vaguely resembles that of the biwa, a Japanese lute, sharper at one end, rounder at the other. But the older name for the lake and the land around it (now Shiga Prefecture) was Ōmi 淡海 < awaumi 'light (= freshwater) sea'. (Compare the native Japanese reading for 湖 'lake' mizu-umi lit. 'water-sea' and the Sino-Japanese compound 淡水 tansui 'freshwater').
The older name, Ōmi, still shows up in a host of local place names—Ōmi-Hachiman (近江八幡), Ōmi-Imazu (近江今津), Ōmi-Maiko (近江舞子), Ōmi-Shiotsu (近江塩津), etc.—but nowadays it's always written as 近江 lit. 'near-bay/inlet/river'. (江 is the e of 江戸 Edo lit. 'bay-door'.) What's up with that?
Well, it turns out there was another notable freshwater lake near Hamamatsu in what is now Shizuoka Prefecture. To the Kinki hegemons of the Nara and Heian periods, it was the 遠つ淡海 Tōtu-ōmi 'far freshwater sea', later shortened to 遠江 Tōtōmi 'far waters', which was also the name of the surrounding province.
The lake closer to the capital was distinguished as the 近つ淡海 Tikatu-ōmi 'near lake', shortened in writing to 近江 'near waters', but pronounced simply Ōmi, since it was, after all, The Lake (like 'The City').
Nowadays, the two lakes are no longer sibling rivals. A major earthquake in 1498 breached the narrow shoreline that separated Lake Tōtōmi from the ocean, leaving its waters brackish, though still very productive. It now goes by the rather prosaic name Lake Hamana (浜名湖 Hamana-ko 'shorename-lake'), while Lake Ōmi sports a more poetic moniker, Lake Biwa: 琵琶湖 Biwa-ko 'lute lake'.