If it's just sitting there that isn't optimum though, eventually it may vibrate and chafe through, if it's mounted then it ought to last until something damages it, like the
water in it freezes during
Winter or something.
Engines are often damaged from a muffler overfilling with
water and water getting into the
engine, but that is not a fault of the muffler, it's from usually either over cranking an engine that doesn't start, or possibly water being forced into the
exhaust from outside of the
boat.
But that is not a muffler fault