What would be different about a heavy-water ocean world?
For one thing, it would be hugely more unlikely to exist.  We are more likely to have worlds where there is a little more deuterium in the hydrogen, not 100%.

Most of the hydrogen in our Universe is protium (hydrogen-1) because the vast majority of unfused nuclear particles in stars are protons.  Neutrons must either fuse with something or they decay to make protons with a half-life of ten minutes.  Deuterium and tritium are formed initially when the protons are fused, but they are just intermediate towards the formation of more stable helium-4 nuclei (alpha particles) and so do not accumulate to large concentrations.

If there were a heavy water world, life would have to adapt to it.  Prolonged consumption of pure heavy water is deadly to life on Earth.