What I think makes sense
I'm ignoring for now the effects of the liquid phase, let's just go for the UV-vis spectrum of CO$_2$. For me this means electronic excitations. Somebody will have measured/calculated those, so let's look for papers. When I search for "electronic excitations CO2" in google scholar, I find this paper: "Vibronic effects on the low-lying electronic excitations in CO2 induced by electron impact" where the introduction states:
The low-energy part of the UV absorption spectrum of the molecule consists of two diffuse bands with maxima at excitation energies of E ∼ 8.4 and 9.3 eV.
and I think that's all we need to know. We can work with the idea that the molecule is so small and simple, and its bonds so strong, that the lack of absortion in the visible spectrum is not an approximation and instead the result of a lack of energy levels in that region of the spectrum. No levels at the right energy above the ground state means no resonance condition means no absortion of visible light.
But, there is this UV absortion. So what color does that leave us with? 9 eV, per the NIST converter are 7.26 x 10$^6$ m$^{-1}$. The inverse of 7.26 x 10$^6$ m$^{-1}$ are 1.38 x 10$^{-7}$ m, or 138 nm. This is, as indicated in the text, in the UV region (<380 nm) but not too far from visible. Since they are diffuse bands, we can assume they are rather broad and could contribute very weakly to some residual absortion in the visible region. The title of the paper also gives us an indication on how this energy region can be reached: through vibronic interactions, i.e. 70
mixing between electronic and vibrational transitions. Above certain temperature, excited vibrational states will be populated, and a (quite forbidden but not zero-probability) transition will happen between the ground electronic state with high vibrational excitation and an excited electronic state with lower vibrational excitation.
The short answer, as for anything that absorbs very faintly in the UV, is that the color would be very faintly yellow.
A revised version of this answer would take into account the effects of aggregation. Surely more interesting things (a richer spectrum of electronic transitions) can happen when CO$_2$ molecules are bunched at high pressure. However here the temperature/pressure regime that resulted in an ocean of CO$_2$ would probably be critical.
Also, what color is this ocean's floor? If the absortion is weak enough, this may be the dominant color as seen from space anyway.
If one absolutely wants to find a spectrum
If hard-pressed to find a spectrum, one could resort to this theoretical simulation: "An instantaneous normal mode theory of condensed phase absorption: The collision-induced absorption spectra of liquid CO2" (<PERSON>, J. Chem. Phys. 107, 5635 (1997)). The positive side here is that collision (and thus condensed matter, in this case liquid phase) effects are taken into account and indeed the topic of the paper, with "liquid CO$_2$" being in the title. The negative side is that I'm way less sure of what is happening here and how much we can trust what we're seeing. The authors do acknowledge that
"Unfortunately, experimental data for the fundamental absorption line-shapes at the state point studied here are not, to our knowledge, available. In fact, we were unable to find band shapes for fundamental absorptions in neat liquid CO2 at any state point, although the corresponding data for CS2 is available."
That having been said, there goes a spectrum together with the corresponding caption:
"The bend absorption spectrum is presented. The Fourier transform of the MD dipole-dipole correlation function is presented for comparison with the INM spectrum." 70