Which naval-history subjects are hardest to research?

What are the most difficult topics in naval history to research, and why?

Diplomatic work, ordnance factories, and armour works are especially difficult because tight security often left incomplete surviving records.

Subjects involving diplomats, ordnance works, or armour works can be unusually hard to reconstruct. These institutions guarded information closely rather than leaving an open and complete paper trail. Records were often deliberately discarded, which compounds the historian's problem. The resulting archival silence reflects institutional security practice rather than a lack of historical importance.