Even a cubic foot of water would be well under 100lbs. I doubt a cubic foot of mussels + water to fill the rest could weigh a ton.

It may be possible for (especially stationary structures) to accumulate enormous columns of biofouling materials, but for such a layer to reach > 20 feet thick (possibly closer to 50 feet without counting the interstitial water) sounds unreasonable. But I'm not knowledgable in this area.