Hi des_ba,
Unfortunately, the Landsat data does not provide a land-water-mask.
To solve this problem, you can use some pixel identification too upfront and at the result to your data. Then you can use it in the valid pixel expression of C2RCC.
Please see theses posts.
You could also use Idepix for Landsat 8.
For S2 MSI such expression on the radiances is defined. But not for L8. I will check why and with the next updated we might provide a default expression.