This seems like transparently unwise behavior for people such as we — residents of a fragile spaceship — to engage in.
>this can't happen if aquifer is already under the sea.
Sure it can: a fresh[er]water spring venting into salt water is still a spring. These types of fresh/salt transitions are seen by divers all the time.
These springs and "meetings of the waters" are often hot beds of life underwater, and shutting them down will have unknown hydrological and biological consequences.
>in this case it will be seafloor subsidence, which will have much smaller impact than land subsidence has.
How do we know that? Just because the impact is less obvious or "more distant" doesn't mean it's less costly.
>also huge profits for the governments taxing these companies, and for people buying the resource cheaper.
Taxes? Lower prices? Sounds like someone's not optimizing for Shareholder Value, citizen. :-\
[Un]fortunately, real businesses do. They use regulatory capture to dodge taxes or otherwise avoid contributing back to the public coffer (see the numerous stories of aquifers given away "for a song" to private companies) and they use anticompetitive monopolistic/oligopolistic practices to avoid lowering prices. These practices are economically inefficient (ie they make the society poorer overall, and by utilitarianism reduce the society's overall happiness), yet they are common.
So I question your claim of automatic huge profits for governments and consumers. It could be that all the profits go to the companies, and any economic profits for third parties get cancelled out by the economic losses incurred by additional income inequality.
Not all profit is profitable. If I give an extra dollar to the local warlord, have I made the society richer? Happier? Free-er? Does it make any difference if the local warlord calls itself "Nestle?" (actually that's not quite fair: Nestle has a body count most warlords would give their left arm for, but that's a separate issue)