Seems kind of strange to put the whole thing underwater if they're going to pump seawater through internal heat exchangers. At that point there's not a lot of difference between putting the "submarine" underwater and leaving it out of -- but near to -- the water and pumping seawater through the same exchangers. With the added benefit of people being able to access things if something goes wrong.
Their public statements so far indicate that, on the prototype, essentially nothing went wrong in a couple of years (a small number of board failures).
On the seafloor, they get constant low temperature water with ~0 external pipe and no head to pump against, so it might not obviously be better to have access.
- there are issues with using water for heat exchange and that affecting flora/fauna. maybe the impact can be spread out more at the bottom of the ocean than for on shore locations
- pumping water all the way on to land and back out can take a lot of energy (water is heavy)
- coastal land is generally expensive, certainly more than seafloor which is free(?)
- coastal land is subject to storm surge in a way that deep seafloor is not
