The ocean's are great stores of CO2, especially at the high pressure seen at depths of over 1000m, or so I am informed. So are they fizzy down there, whats it like for a fish, do they basically live in a soda stream?

Up near the surface there is too much exchange with the atmosphere for the CO2 concentration to rise too high, At the depths where there are higher concentrations there is no question of fizz, because the pressure is far higher than what you might get in a soda bottle. Fish would in any case avoid strong concentrations, just as we would choke if we got a face-full of strong CO2 (trust me!  I have tried!) Where there really is a high concentration, it actually can dissolve CaCO3 sea shells.

The concentrations can however get far higher than  is necessary to cause fizz if you bring the deep water to the surface. There is enough CO2 down there to support a fountain if you lowered a pipe far enough. It has even been suggested that we could use that as a source of power.

Not a bad idea, except that it would be the most CO2-intensive source of power on the planet!

Cheers,

Jon