The ability to know when someone will die is a heavy and disheartening burden to carry, but at least she had the ability to prevent some deaths in her 10 years of living.
Seeing alone is not enough. That Marisa is probably as good in understanding death flag as Keima is in understanding romance flag. She understand the common deadly patterns (e.g: "lets get married when I came back from this war") and what event that will cancel it.
Marisa realized it's possible after she broke the flag and saved that Deibu from falling from the cliff; she's been pushing events that helps her fellow clan members avoid death without them even realizing it.
Frankly, it's amazing that death has been leaving her alone for 10 years, knowing how Death doesn't like his job disturbed in Final Destination. Alas... Gensoukyou's god of death is Komachi, so... ^^;;;