Could be a number of things here are some possibilities I've come across on the forums mostly. Many here have lots more experience so trust them over me, but I figured I'd share a response to give you some things to check next.
Many people will ask is it raw water cooler or fresh water? Alpha 1 or Gen 2?Fresh or salt water? How old and how long has this problem occurred and is the boat new to you or you've owned it a long time? How long since you inspected or changed your risers and exhaust manifold?
Impeller/sea pump issues, clogged risers, clogged intake tube between outdrive and housing, collapsed/kinked/clogged tube somewhere along engine and water flow, exhaust flapper or restriction issue, water pickup tube in outdrive issues, impeller housing worn or inferior impeller parts, etc.
Hope this helps a bit.
Might also be worth shooting the engine with an IR thermometer gun but if you are seeing 160 while cruising my guess is your temp gauge is fine.