Re: 79 mercury 1150 115 horse outboard
OK, simple issue - only four causes that I know of.
Was the starter taken apart and reassembled with the magnet turned upside down? This causes starter to spin the wrong way., and bendix cannot engage flywheel.
Is Bendix dirty and causing gear to stick? If so, clean bendix and reoil.
Are battery connections clean enough to let starter spin fast enough to engage flywheel? Cleaning the connections and inspecting the battery cables is the remedy. You can have corrosion under the battery cable insulation. Look for bulges.
Maybe starter is just bad?