What should be the advantage over a minefield?

Mines are also bad for your own ships.

It's cheaper, faster

Not so sure about that. Nordstream 2 took only two years to complete. If you deploy a minefield of 100km x 10km you need around one million mines. If one mine takes 6 minutes to deploy, then you need 10 years for all of them (or one year with ten ships). Then even the discount version of mines would cost at least one billion Euro plus they have a "best before date" because of seawater corrosion.

the bubbles take some time from the ground

Even fast ships are relatively slow and since the pipelines are close to the shore, you can detect them visibly. That shouldn't be a problem. The pressure from the turbines simply has to be strong enough.

And maybe the Chinese already have hovercrafts as landing vehicles but I don't know.

Hovercrafts would either likely sink as well, or they would be easy targets because of a lack of armor.

Thanks for the input.