What would stop heists on the high seas? For example, I could imagine someone breaking into a container with "valuable" goods (remember a lot of these pirates come from impoverished backgrounds, so being able to flip cargo still net a nice profit).
With no humans on board, then there is no incentive not to use deadly force for disabling critical subsystems.
I suspect with drone ships and the lack of crew liability, there will arise contract security companies with pre-positioned ex-seal teams in hotspots to deal with piracy.