Halve avocado and remove pit. Scoop out flesh and add to the bowl of a food processor or a blender.
Add the garlic, olive oil, and lemon juice. Purée until well chopped but not completely smooth. Add half the salt and pepper and purée again briefly.
While the processor is running, pour water through the feeder tube, blending until smooth. Add more water as needed to reach the desired consistency. Use less water for a dip; more water for dressing. Taste and adjust seasoning.
Pour into an airtight container and store in the refrigerator. Use within 5 days.