The CN-235 is fine for ISR down the Somali Coast (assuming the owning Nation is happy for it to fly there), and also patrolling the Internationally Recognised Transit Corridor. However, seraching for motherships, dhows and the go-fasts that operate from them throughout the vastness of the Indian Ocean, you need a platform that can react quickly to get to a hotspot of pirate activity, can reliably detect small targets on radar, can take good photographic images that can be used as evidence in court later, and can stay there as long as possible to reduce the AOP for when the surface asset comes steaming over the horizon (which is often 24 hrs+).
The RN 2* that was running the EU Op (Atalanta) when I was there, was very vocal whenever anyone from the UK Govt came to visit about the fact that he would have loved to have the Nimrod available to him!