This means that there's no sense in trying to carbon date a salt-water fish, a whale, a sea-otter, and so forth.
Actually we date marine organisms all the time. But we do have to include a marine correction factor, which varies from area to area. This is called Delta-R.
For the area I work in that figure is 22535 years. This is in addition to a worldwide marine correction factor that I think is something like 400 years.
But it is always good to test these various corrections against one another.
We encountered a feature in an archaeological site a few years ago consisting of three nestled abalone shells (face up), and in the upper shell we found both halves of a large mussel shell and charcoal. Capping this we found two more abalone shells, face down.
We dated one abalone shell, one mussel shell, and the carbon. When all corrections and calibrations were applied the three dates spanned 14 years.