A little co2 can be a good thing as Bruce said it forms Carbonic acid when it combines with water and it can free Ca and allow it to be assimilated by invertebrates, but the levels we have now are becoming dangerous.
Co2 and O2 do not compete for space in an aqueous solution. This means that you can have water that's saturated with O2 and this will not affect the amout of co2 that can be dissolved in water. Theoretically you can have water saturated with O2 and CO2 at the same time, however this really isn't possible in nature, or atleast on earth... or atleast not yet
If it ever gets that bad that that's possible we'll all be dead by then.
As far as the oceans changing temperatures dramatically, a few degrees is all it takes to wipe out coral populations.
So can saltwater hold more or less CO2 than fresh... Very good question! Wish I knew! Just brainstorming here but since saltwater is more dense then perhaps CO2 isn't as easy to dissolve than it it is in fresh. Keep in mind that co2 is indeed easily dissolved in water compared to O2 which is relatively hard to dissolve.
If I was playing who wants to be a millionaire, I'd say that Co2 is easier to dissolve in fresh water than salt water.
Another idea is aquatic plants. They require co2 ppm of about 10 times that of normal air. If the limiting factor of CO2 is taken care of by man the plants will grow at a rate that is not sustainable with the limited nutrients available in the water column (and to a lesser extent the substrate), assuming that industrial and septic runoff is not a factor in this EQ. the plants will grow at a increased rate, use up the nutrients, die and release it all back into the atmosphere. Since all the nutrients are gone, nothing in the water will grow back (or atleast not porportionally) to re-utilize the co2 produced from the decayed plants, compounding our problem.
Without the aq. plants, species will starve, die, and release more co2.
Hey, just brainstorming. I could be wrong lol