Because the original joke and your comment were both premised on how we valuing things / create valuations for things, and at its core, this is was why the field of environmental economics exists.
Prior to environmental economics, we had capitalists saying non-market environmental goods (e.g. the existence/conservation of whales, conserved lands / national parks, etc) are not worth anything bc they don't have a market value. And on the other extreme you had environmentalists who were claiming that these goods are priceless / immeasurable / infinite value.
The chasm between 0 and infinite didn't leave much room for these two sides to compromise. And so the field of non-market valuation was born. It attempts to value non-market goods by calculating the positive and negative externalities of these goods. Thus, in the case of the Great Barrier Reef, we could attempt to calculate how much effect this pumice raft would have on the health of the reef and then we would calculate how much value the reef provides through its biodiversity.
Even if we are extremely underestimating the value, a lower bound estimate is still better than is being valued at 0.
And then we can estimate the internalized market value of this good. i.e. what would the market pay given current market prices for pumice? And what is the cost of harvesting this pumice versus other pumice. Because other pumice doesn't have these potential environmental benefits. It simply sits on the ground or underground close to the surface.
So if the costs of these 2 sources of pumice are close but the benefits of the sea raft pumice is much higher, than the government should put a Pigouvian tax on the raft pumice to discourage its harvesting (or simply ban such harvesting).
But I would wager this is all a fruitless conversation because I bet the cost of harvesting the sea raft pumice is much higher than the cost of harvesting land pumice.
Pumice Prices (closer to retail):
- $7-18 per pound for cement mix
- $2-9 per stone for scrubbing, cleaning, and exfoliating
- $8-16 per pound for powder use in makeup and exfoliating body scrubs
Given that pumice is mined via open pit mining, it is very cheap, economical/efficient, and environmentally friendly:
>The mining of pumice is an environmentally friendly process compared with other mining methods because the igneous rock is deposited on the surface of the earth in loose aggregate form. The material is mined by open pit methods. Soils are removed by machinery in order to obtain more pure quality pumice. Scalping screens are used to filter impure surficial pumice of organic soils and unwanted rocks. Blasting is not necessary because the material is unconsolidated, therefore only simple machinery is used such as bulldozers and power shovels. Different sizes of pumice are needed for specific uses therefore crushers are used to achieve desired grades ranging from lump, coarse, intermediate, fine and extra fine.
I don't think harvesting from the ocean would be cheaper than using simple bulldozers to fill trucks. Given that it then needs to be dry, the energy costs of ocean pumice would be much higher (even if the initial harvesting could be comparable in cost).
Thus, no intervention is needed because no one would even think about harvesting this pumice...
 A simple example is using the travel cost method from non-market valuation to value a national park. Even though parks are virtually free (visitors pay a nominal fee), we can add up all the money someone spends to fly their family to Yellowstone and then rent a vehicle and pay for hotels and say all that money spent is a lower bound