Cessna 172 pilot reported an engine malfunction during cruise. The pilot diverted and landed safely.

Narrative

I was 400 feet along the ocean; parallel the shoreline underneath the bravo. I made a turn to 250 once the shore line ended and was given an altitude clearance to maintain at or below 1400. When I went to put full throttle for the climb; I noticed my engine rolling back and the max RPM I was able to attain was 2200rpm. I announced on ZZZ approach that I needed to amend my destination to the nearest possible airport due to engine roughness after I followed all my checklists in order to trouble shoot the problem. ZZZ approach vectored me to turn northbound towards ZZZ where I struggled to climb to 1100 feet for the duration of flight in order to maintain clear of any obstacles. I was able to tell ATC how many soul and time remaining on fuel onboard and complied with all their vectors and instructions. I was able to land on the runway at ZZZ safely while the fire ambulance; police and rescue squad were waiting for me. I gave my statement and information to the police; denied medical; and spoke to the FAA advisor in which whose number was given to me by the ZZZ airport manager. The plane is currently undergoing maintenance to determine what happened; and to repair the issue while we wait for a release clearance on the aircraft. No injuries or substantial damage was caused from this; otherwise a normal flight. I learned the value and important of your checklist and troubleshooting.