The Marines are one of the Three Great Powers in the world of One Piece. They are affiliated with the World Government, and their main goal is to stop all the criminals in the world. They have been helping the World Government by getting rid of people, who could pose a threat to their authority. The Marines are led by the Fleet Admiral, and he has complete authority over every soldier.
The current Fleet Admiral is Akainu, who was promoted after winning the fight against Aokiji. Under Akainu's guidance, the Marines have become much more ruthless, and they have been actively trying to hunt pirates. Over time, the Marines have gained the reputation of being unbeatable, but that is not the case, in fact, there are many One Piece characters who are stronger than the Fleet Admiral and the Admirals. In addition to their lack of power, the Marines are not the saints that they claim to be.
Villains
While the Marines claim that they aim to serve and protect the people, on many occasions, they have attacked and killed innocent people. In the past, when a buster call was initiated on Ohara, the Marines attacked many scholars. Furthermore, when the remaining citizens were evacuated on a ship, Akainu ordered his troops to destroy it. Akainu did not care about the innocent people on the ship, he just wanted to go through with his "Absolute Justice."
When the Straw Hats were still in East Blue, they came across Nezumi, who was an extremely corrupt Marine captain. He was on good terms with Arlong and the latter gave him bribes to cover up his actions. Arlong's relationship with Nezumi allowed him to terrorize the people living in Cocoyasi Village, and without any interference from the Marines. As Arlong's actions were kept under wraps, he only had a bounty of 20,000,000 beris, which would have otherwise been much higher. These are just a few instances where the Marines abused their powers and caused harm to innocent people.
Massively Underwhelming
The Marines have many strong fighters, such as Akainu, Garp, and the three Admirals. Garp is undoubtedly still very strong and probably the only character, who can fight a Yonko. Garp is the most decorated Marine and no one comes close to him. Akainu was humiliated by an old and sick Whitebeard, and there is no way that his power would have increased so much that he could best a Yonko in a fight.
Many fans overlooked Whitebeard defeating Akainu, and they still believed that the Marine Admirals were just as powerful as the Yonko, however, in the Wano Country arc, their headcanon proved to be wrong. When Admiral Ryokugyu was attacking Momonosuke and the Scabbards, Shanks was nearby, and he unleashed his Conqueror's Haki. The burst of Shanks' Haki scared the Admiral and made him stop his attacks. Ryokugyu admitted that he did not want to fight the Red Hair Pirates, and he left. This incident further proves that the Admirals are nowhere near the level of the Yonko.
Using the Shichibukai
Before their dissolution, the Shichibukai used to be one of the Three Great Powers. They were made up of pirates, whom the Marines considered powerful enough to make a difference in important fights. They were offered a lot of amenities by the World Government in exchange for their services, which led to a lot of pirates actively trying to get a hold of this position.
However, the system was flawed and the Shichibukai often abused their power. For example, Crocodile in Alabasta, and Doflamingo in Dressrosa. Crocodile nearly took over the entire nation of Alabasta, while Doflamingo successfully ended the reign of Riku. In both of these instances, the Marines tried to cover up their actions, but after Doflamingo was exposed by Luffy, they could no longer cover their tracks anymore. Many people, including Admiral Fujitora were against the Shichibukai system, and it was finally abolished in the recent Levely.
Why They Still Exist
One of the most common questions, that the fans ask is: if the Marines are so weak, then why don't the Yonko destroy them completely? There are numerous reasons why the Marines have managed to stay in power, and one of them is the animosity between the Yonko. The Yonko do not like each other, and they are always on the lookout for opportunities to get the better of one another. The fierce competition between the Yonko wouldn't allow them to team up against the Marines, but if two Yonko tried to get in contact, the Marines would try to do everything in their power to sabotage their meeting. This has happened twice in the series so far.
The first time that two Yonko met each other was just after the Enies Lobby arc. The Marines had tried to stop them from meeting each other, but they failed. Luckily for the Marines, Shanks and Whitebeard were not planning on joining hands, however, they were not so lucky the second time, when Kaido and Big Mom decided to form an alliance. When the news about the formation of a possible alliance between the two Yonko, the Marines were left shocked.
They had no idea how to deal with the alliance. Luckily for them, the members of the Worst Generation banded together and took down the Yonko. If Big Mom and Kaido had been allowed to run, it wouldn't have taken them to turn the world upside down. So, it is extremely clear that the Marines are heavily reliant on luck, and if the Yonko wanted, they could easily join hands and defeat them.