If the hose were leaking, wouldn't water be flooding the bilge instead of the livewell?
So anyway, your livewell overflow is an open hole in the side of the well with a hose leading to a thru-hull fitting through the transom, which is underwater when the boat is floating?
If the overflow hole is lower than the surface of the lake, water will pour in, until the level of water in the livewell is even with the surface of the lake.
Couple ways to prevent that from happening: make the boat float higher in the water (by throwing something heavy overboard), or get a drain plug that fits the hole.
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