A now defunct website listing Soviet Northern Fleet aircraft losses in WWII (
http://www.eismeerfront.com/eis_html/poteri.htm) had two GST (Soviet code of the Catalina) of 118 MRAP were lost on 27 July 1942 in "Karmakuly Bay", so this seems to confirm the claims of U-601. Except that if one of the GST was listed as "sinking after being shot at by enemy submarine", the second is listed as "sinking, shot down by aircraft".