I'm familiar with the processes in the body, but that is a different thing than sea water (less saline). Are you talking about the fraction of CO2 not in solution? Else algae wouldn't be able to use it.
Concentrations of molecules and ions may differ, but the varius equilibia reactions will still happen in essentially aqueous solutions. Water still has minute amounts of hydronium and hydroxide ions in equilibrium. Adding CO2 makes it acidic, but just weakly. It doesn't completely dissociate, as opposed to strong acids like sulphuric or nitric acids which almost completely dissociate for all intents and purposes. That's why when CO2 is involved it's called a buffer system.