Good question. Salt does affect the rate at which CO2 can go into water by affecting its solubility. The higher the level of salt, the lower the solubility.
Because of this, you might expect that lakes would have a higher level of CO2 (per unit volume) in the water than the ocean, but this is actually quite a complex question. The amount of CO2 in the water will be affected by the currents in the water, which may move surface waters deeper or bring waters up from the depth with different amounts of CO2, it will depend on the amount and type of biological activity in the water (aquatic animal life emmit CO2, and aquatic plant life absorbs CO2, just like on land), it depends on some quite complex biological and chemical processes, for instance where carbon is taken up and used by tiny creatures to make their shells, and it will depend on the conditions at the sea surface (things like the wind speed and the waves) which affect the transfer of co2 into or out of the water.
These are just some of the factors involved, so it is hard to say whether a particular lake or ocean, or lakes or oceans in general, have more CO2 (per unit volume). This is one of the things that scientists worldwide are investigating by making careful measurements and by increasing our knowledge of the physics, chemistry and biology involved.