Meaning "sea"
* Marine: Relating to the sea
* Maritime: Relating to the sea, especially its navigation or commerce
* Mariner: A sailor
* Submarine: A vessel that travels underwater
* Mariculture: The cultivation of marine organisms
* Mariner: A sailor or seaman
* Marquee: A large canopy or awning, often used for entranceways, originally used for ships
Meaning "to harm, injure, or spoil"
* Mar: To spoil or damage (often used in the phrase "marred by")
* Martyr: One who suffers death for a cause, often religious
* Marauder: A raider or plunderer
* Marplot: One who interferes with or spoils a plan
* Marmoreal: Made of or resembling marble (marble was used in ancient Rome for statues, which were often damaged by time)
Other
* March: To walk in a military manner, derived from the god Mars, the Roman god of war
* Marsupial: A mammal that carries its young in a pouch, such as a kangaroo. The Latin word "marsupium" refers to a pouch or bag.
This is not an exhaustive list, but it shows the versatility of the root "mar" and how it can be used to create words with a variety of meanings.