renault megane 2004 1.6 ?

my renault megane has developed a problem with the battery it started about 3 days ago when it rained very bad the battery went flat over night i checked the ecu which was full of water so i removed and dried over night replaced the ecu jump started car ran fine for 3 days then all of a sudden went flat again in the space of three hours please can anybody shed light on the situation