If the water drops too low, the battery will fail to function. Older style batteries have cell vent caps. Low maintenance batteries have a pry-off cell cover. Remove the cell vent caps or use a screwdriver to pry off the cell cover to inspect the electrolyte level. The electrolyte level should be well above the plates.
Most manufacturers have a guide ring built into the top of the case above the cell.
The electrolyte level should be filled to the level of the guide ring. If the electrolyte level is low, add water to the cell. Because regular tap water may have a high salt and mineral content, you should use distilled water. The water should be added with a plastic or rubber tool made for battery filling, not a metal funnel. A metal object could cause a short between the plates.