Marmoush scored twice in the space of three minutes early in the second half before substitute Rayan Ait-Nouri added a third in stoppage time, helping City overturn a halftime deficit in front of more than 50,000 fans at Seoul World Cup Stadium.
"Overall, it was a very good test against one of the best teams in Europe," Maresca said. "Credit to the players for the effort they have done. From tomorrow, pre-season is finished and the real business starts."
City dominated possession and created the better chances but fell behind against the run of play when Jorge Dominguez scored from close range shortly before halftime.
The Premier League side finally made their pressure count in the 57th minute as Semenyo raced down the left and crossed for Marmoush to slide home the equaliser. Two minutes later, the pair combined again, with Semenyo delivering another low cross that Marmoush converted.
Semenyo finished the Asia tour with four assists in three matches, while Savinho also impressed on the opposite flank. Ait-Nouri completed the scoring late on, but Maresca said City still needed greater efficiency in front of goal.
"We need to be clinical. It's something we need to improve," he said. "Slowly we are getting there and with Erling (Haaland) we can be even more dangerous. Today Omar scored twice. This is a boost."
Maresca also said he was encouraged by the progress made during his first three matches in charge after succeeding Pep Guardiola, with City ending their Asia tour with a draw against Inter Milan, before a penalty-shootout defeat, and victories over a K League All-Stars side and Atletico.
They face Arsenal in the Community Shield on August 16th.