Author says:

It was not the case that they had not drunk any water for three days and nights. Access to water had been closed for three days and nights, but during this time they had been able to get some water on one or two occasions, including the night of 'Ashura', when they had taken bath and washed their bodies.

Even with such narrations which talk about the presence of water in Karbala until the night of Ashoor, one cannot deny the fact that Imam Husain (as) was martyred thirsty. 

Hadith 1:

We also have the following narration which indicates that water was not available for 3 days:

أبو مخنف: روى بعد ذكر شهادة علي الأكبر

ثم أقبل الحسين (عليه السلام) إلى أم كلثوم، وقال لها: يا أختاه! أوصيك بولدي الصغير خيرا، فإنه طفل صغير، وله من العمر ستة أشهر. فقالت له: يا أخي! إن هذا الطفل له ثلاثة أيام ما شرب الماء، فاطلب له شربة من الماء. فأخذ الطفل وتوجه نحو القوم وقال: يا قوم! قد قتلتم أخي وأولادي وأنصاري، وما بقي غير هذا الطفل، وهو يتلظى عطشا، فاسقوه شربة من الماء.فبينما هو يخاطبهم إذ أتاه سهم مشوم من ظالم غشوم، وهو حرملة بن كاهل الأسدي، فذبح الطفل من الوريد إلى الوريد، أو من الأذن إلى الأذن، فجعل الحسين (عليه السلام) يتلقى الدم حتى امتلأت كفه ورمى به إلى السماء

Abu Makhnaf narrates after the Martyrdom of Ali al-Akbar (as). Then Imam Husain (as) came to Umm-Kulsoom (sa) and said to her, "O Sister, I recommend you (as a will/Wasiyah) to do good regarding My small Son because He is a very small child and He is only 6 months old." 

Then Umm-Kulsoom (sa) said to Imam Husain (as), "O Brother, verily for 3 days this small child has not drank water, so ask for water for him.

So Imam Husain (as) took the child and faced the enemies and said, "O People, you killed My Brother, My Children and My helpers and none remains except this child and He is burning due to thirst, give Him a drink of water. And while the Imam (as) was addressing them, He was shot by an horrible arrow from the ignorant tyrant Harmalah bin Kahil al-Asadi (curses be upon him), which slaughtered the child from vein to vein or from ear to ear. Then Imam Husain (as) collected his blood and filled it in his hands and threw it towards the sky."

[Source: The Encyclopedia Of Imam Husain's (as) Words Compiled by: Behrul Uloom Researh Center, Pg. 574-575]

Online link: shiaonlinelibrary.com/الكتب/1691_موسوعة-كلمات-الإمام-الحسين-ع-لجنة-الحديث-في-معهد-باقر-العلوم-ع/الصفحة_567

One should ponder and think, If there was so much water available on the night of Ashoor such that the companions of Imam Husain (as) took bath and washed their bodies then why not preserve some water for the next day?

Hadith 2:

قَالَتْ سَكِينَةُ لَمَّا قُتِلَ الْحُسَيْنُ ع اعْتَنَقْتُهُ فَأُغْمِيَ عَلَيَّ فَسَمِعْتُهُ يَقُولُ‌

شِيعَتِي مَا إِنْ شَرِبْتُمْ رَيَّ عَذْبٍ فَاذْكُرُونِي

أَوْ سَمِعْتُمْ بِغَرِيبٍ أَوْ شَهِيدٍ فَانْدُبُونِي

فَقَامَتْ مَرْعُوبَةً قَدْ قُرِحَتْ مَآقِيهَا وَ هِيَ تَلْطِمُ عَلَى خَدَّيْهَا وَ إِذَا بِهَاتِفٍ يَقُولُ‌

بَكَتِ الْأَرْضُ وَ السَّمَاءُ عَلَيْهِ

بِدُمُوعٍ غَزِيرَةٍ وَ دِمَاءٍ

يَبْكِيَانِ الْمَقْتُولَ فِي كَرْبَلَاءَ

بَيْنَ غَوْغَاءِ أُمَّةٍ أَدْعِيَاءٍ

مُنِعَ الْمَاءَ وَ هُوَ عَنْهُ قَرِيبٌ

عَيْنٌ أَبْكَى الْمَمْنُوعَ شُرْبَ الْمَاءِ

Narrated that Sakinah (Sukaynah) bint al-Husayn (عليها السلام) said,

When al-Husain (عليه السلام), was killed, I embraced/hugged Him, and I fainted, so I heard Him say, 

"My Shia, whenever you drink reviving water remember Me,

Whenever you hear of a stranger or a martyr, so cry for me"

Then she got up frightened, while her eyes were injured, and she was slapping her cheeks.

At that time someone called out saying:

"The earth and the skies cried over Him,

With plentiful tears and blood,

Crying for those killed in Karbala,

Between a noisy crowd of illegitimate people,

Water was denied to Him while he as near to it,

O eye, cry for the one who was denied a drink of water."

[Source: Misbah Al-Kafami, Pg. 741]

Hadith 3:

Anther similar narration from Imam Sadiq (as):

قال الصادق (ع) " اني ما شربت ماء باردا الا وذكرت الحسين

Imam Sadiq (as) said, "Verily, I didn't dring cold water, but that I remembered Husain (as)."

[Source: Misbah Al-Kafami, Pg. 741]

Hadith 4:

قال اسقوني شربة من الماء فقد نشفت كبدي من الظّمأ

Imam Husain (as) said, "Quench my thirst by a drink of water, as my heart/liver has dried up due to extreme thirst"

[Source: Asraar Al-Shahadah, Pg. 409 / Al-Muntakhab Le Tareehi, Pg. 439]

The above words indicate that the thirst of the Imam (as) would be at least of a few days.

Hadith 5:

حدَّثني الحسن بن عبدالله بن محمّد بن عيسى ، عن أبيه ، عن الحسن بن محبوب ، عن العَلاء بن رَزين ، عن محمَّد بن مسلم ، عن أبي جعفر عليه السلام قال : إنّ الحسين صاحب كربلاء قُتل مظلوماً مَكروباً عَطشاناً لَهفاناً ، وحقٌّ على الله عزَّوجَلَّ أن لا يأتيه لَهفان ولا مَكروبٌ ولا مُذنبٌ ولا مَغمومٌ ولا عَطشانٌ ولا ذو عاهةٍ ثمّ دعا عنده وتقرَّب بالحسين عليه السلام إلى الله عزَّوجَلَّ إلاّ نفَّس الله كُربتَه ، وأعطاه مسألتَه ، وغفر ذنوبه ، مَدَّ في عمره ، وبسط في رِزقه ، فاعتبروا يا أولي الأبصار

Narrated that Imam Baqir (as) said, "Imam Husain (as) the Master of Karbala was slain while he was oppressed, distressed, thirsty and aggrieved. It is indeed fitting that every aggrieved, distressed, sinful, saddened, thirsty and ill person who goes to his Ziyarat and prays near his grave, seeks proximity to Allah through him, will have his grief removed and Allah will fulfill his needs, forgive his sins, prolong his life and increase his sustenance. “So take a lesson, O those who have insight!”

[Source: Kamil Al-Ziyarah, Chapter. 69, Hadees. 5]