To rewater is to water again—typically plants or soil that have dried out since their last watering.
The word appears in gardening contexts when something needs a second dose: I'll need to rewater the seedlings this afternoon.
Less commonly, it describes restoring water to something that was drained or dehydrated.
1. plants — irrigate, Supply land with water; sprinkler (Water-dispersing lawn fixture); drip (Liquid falling slowly); moisture (Small amounts of liquid); soil · houseplant · garden · potted · wilted · drooping · parched · seedling · planter · roots
2. replenish — refill, Restoring spent contents; restore (Return to earlier state); top-up (Adding to restore fullness); refresh (Restore energy or newness); reconstitute · rehydrate · resoak · dampen · saturate · soak · steep · soften · absorb · replenishment