Relevance of Water

Relevance in Veda

Please see explanation of pancha bhuta in the associated blog

Why water is used for various vedic rituals

1. Water is easily available resource so it can be used with minimal or no cost
2. Water indicates life Example: scientists first search for water to get indication of life. 
3. Water is the symbol for purification. hence symbolically for all purification acts water is used including inner purification
4. Invocation of lord in water would increase respect for the environment and hence can never pollute rivers and other water bodies

Spiritual importance

According to veda, water has high spiritual importance as a universal purifier. Of course, Agni is the supreme symbol of purity and purification, but for obvious practical reasons, water is used more intimately to purify.  Water is also called as Apa

In the Vedas, Agni is called jAtavedas and vaishvAnara, which signify omnipresence and omniscience. So it is Agni present in water that gives water the quality of purity.  
Based on few vedic references, water purification capablity is derived from agni.

Vedic References

In Rig Veda 2.1.1:

त्वमग्ने द्युभिस्त्वमाशुशुक्षणिस्त्वमद्भ्यस्त्वमश्मनस्परि । त्वं वनेभ्यस्त्वमोषधीभ्यस्त्वं नृणां नृपते जायसे शुचिः ॥

“You, O Agni, are present in days, in water, in rocks, in forests, in plants, in human beings, and you are pure”

Rig Veda 4.40.5
हंसः शुचिषद्वसुरन्तरिक्षसद्धोता वेदिषदतिथिर्दुरोणसद् । नृषद्वरसदृतसद्व्योमसदब्जा गोजा ऋतजा अद्रिजा ऋतम् ॥

 Similarly  “The Swan (Agni) is present is water, as a Vasu in the atmosphere, as a hotr in the altar, as a guest in the domestic hearth. He is present in humans, gods, space, and constantly born out of waters, earth, cosmic dynamism, mountains, he is the cosmic truth.”

Based on above verses it is clear that water gets properties of purification from the agni as element in the water

Now coming to special relevance of water. Following are few verses from sandhya vandanam

आपो हि ष्ठा मयोभुवः ता न ऊर्जे दधातन । महे रणाय चक्षसे ॥ १

“O waters! you are the source of well-being, and the provider of vigor, for the sake of great vision.”

यो वः शिवतमो रसः तस्य भाजयत इह नः । उशतीरिव मातरः ॥ २

“Whichever is your most auspicious rasa (essence), please share it with us. Just like generous mothers.”

आपः पृणीत भेषजं वरूथं तन्वे मम । ज्योक्च सूर्यं दृशे ॥ ७

“O waters! please make my body an armor with your medicinal properties. So that I may keep seeing the sun for a long time.”

इदमापः प्रवहत यत्किं च दुरितं मयि । यद्वाहमभिदुद्रोह यद्वा शेप उतानृतम् ॥ ८

“O waters! please take away all this that is bad in me - my betrayals, sins and untruths.”

Reference

https://www.quora.com/Why-is-Achamanam-performed-at-the-beginning-of-every-puja