Does anyone know what this is? I know this is not a land mammal because [details needed].
A reasonable match for many features, bearing in mind the incompleteness of the skull is that of the South African Fur Seal (AKA. brown fur seal, Australian fur seal).:
Carolyn Stewardson, Tania Prvan, Meÿer MA, Raymond J Ritchie Copyright status unknown, fair usage via Researchgate
Shown here are views of an adult male of 10 years, the parts circled are those present in your specimen. The slight brow-ridge present on yours is just visible on the bottom plate.
The marked difference between your specimen and this one is the presence of sutures between parietal and frontal bones, which seem to be fused on yours indicating an older specimen (perhaps up to 30 years), but:
Suture age was not a good indicator of absolute age or age group
It's also difficult to tell the match of the slope of the skull bones away from the centre line particularly the parietal - which may be a match or not.
Submitted until a better match comes along.