1.Provide source for 80% figure.The figure is bound to be substantial but i am interested in source.
2.Water that flows into the ocean is not waste.It has many practical purpose,the reason i asked what is your definition regarding your unused water.
a. It is important for river ecology
b. It is important for fisheries
c. It is important as Salt water would intrude in riverine areas.
d. It is important as the fall in fresh water in underground aquifers is compensated by sea water thus destroying fresh water source.
e. It destroys deltaic ecosystems.
f. It increases biological oxygen demand along the river length.
Now for your original query as to IWT, Read it first (or at least it's summary)
India has full right over use of water Ravi,Beas and satluj and pakistan over Indus,Jhelum and Chenab.
And now answering your what if Indus water treaty is nullified query
"Assume IWT is nullified"
In this case assess what could be done with each and every river.
1. Indus
This river originates in Tibet and flows in India between Ladakh and Zaskar range.Its major tributaries are Shyok and zaskar in India.It flows through an mountainous cold desert where nothing grows and we cannot exploit it in any manner except for that of hydropower.
2. Jhelum
Originates in India and have kishenganga as major tributary.
This river flows between pir panjal and zaskar range.It's flow could not be diverted because of Pir panjal range and could not be utilised locally as
a. Kashmir does not have high cultivable area.
b.It has temperate climate thus it's agriculture is mainly oriented towards horticulture
3.Chenab
It flows for some part between zaskar and pir panjal range and for some distance in jammu.
It's main tributary is tawi river.
These are the three rivers for which pakistan has a right to use water.The only river we could theoretically exploit even after withdrawing from IWT is tawi river and to some extent chenab.
For lower river we have the right to use them anyway we want and we are using them also.
Punjab and Haryana comes under arid area and after utilizing water for their use there is nothing left to spare.
Had already typed this post so couldn't let it go waste.