I understand it to be C3 because it lacks Kranz anatomy. The C isotope ratio is C4-like due to discrimination of bicarbonate ions rather that C fixation by PEP carboxylase.
I also agree with the above answers. But some sea-grass species has bundle sheath but not obvious in leaf (Thalassia testudinum). I think that the C.serrulata may have inconspicuous bundle sheath. If so it could be C4...please check it.