You say that sometimes your bilge is full, and other times it is fairly dry. You need to first determine if there is a pattern. If it happens after it rains or you wash the boat - well that indicates where it is coming from. One would think that a below the waterline leak would not be intermitant. You need to check frequently and note prior situations. Determining wether this is rain water or a hull leak should not, over a little time, be difficult to determine.