Short answer: Yes—water can be taken in Ekāṣaṇa, but with discipline.

What Ekāṣaṇa means

• One meal, at one sitting, in daylight.

Water rule (tradition-wise)

• Shvetāmbara: Boiled/filtered water is allowed during the day. If you are keeping Chauvihār, then no water after sunset. If you adopt a stricter niyam, you may take water only with the meal.
• Digambara: Water may be taken with the meal. Many follow the rule of only boiled/filtered water during the day; with Chauvihār, no water after sunset. Some stricter practitioners limit water to the meal only.

Good practice

• Use only boiled and properly filtered water.
• Avoid water after sunset if observing Chauvihār.
• Follow the specific niyam given by your Ācārya/Guru for your sect and vow level.

Essence Ekāṣaṇa is about restraint: one daytime meal, and very limited, pure water—ideally only boiled/filtered, and none after sunset if you keep Chauvihār.