Any idea of how water gets in if bellows look good no holes and clamped on
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From the bilge when the plug is left in them or the bilge pump chits out after a big rain.

Hummmm, plug always in boat is on a lift. Maybe automatic bilge pump took a chit over the winter when on lift??

I would check those bellows over real close again also..

If there is water getting in and the oily sludge getting out into the water, then it has to be a bellow that was leaking.
If the bellows were sealed tight and not cracked, it would not get out into the water.