Three-drop water radical, the left-side form of 水. Lakes are bodies of water, so 氵 places 湖 in the broad water family alongside 河 river, 海 sea, 江 river, 池 pond.
Phonetic - 胡 is hú, 湖 is hú, exact match. 胡 itself is a phono-semantic compound (古+月) used in many lake-name compounds. Same phonetic appears in 糊 paste, 蝴 butterfly, 葫 gourd.
