Here in my place there is a shortage of river water during these days because of lack of rain. People are demanding to use sea water where we can so please suggest the ways in which we can use sea water...
To produce fresh water from sea water sustainably (i.e. sustainable in usage, if not in production), I think solar stills are your best bet.
The article "Solar Thermal Technologies for Seawater Desalination: state of the art" by Jenny Lindblom (PDF) states that a single-layer solar still can produce up to 6 litres per day, per square metre. More intricate (double-layer) designs may provide better insulation and consequently yield even more water.
For long-term improvement, I would also consider investigating why the river now carries less water than before. An forest upstream may be able to store a lot of water, induce rain and stabilize the flow of the river.
Irrespective of the abundance or shortening of water, it would also be a good idea to reduce the use of water in the household & reuse grey water where possible.
Seawater itself may have other applications as well. One example would be a seawater greenhouse.
What do we use water for in every-day life? And can we use untreated seawater instead? Because: untreated seawater is what the PO's is supposed to use instead of fresh water. :)
- Bathing. Maybe Either in the sea or in the bathtub. Yes, if you want to cool down and don't mind the salt. No, not if you want to get really clean. Or should we update our definition of 'clean'?
- Cooking: **No / Maybe ** Interesting. Food is salty of savoury and boiling it kills bacteria, but might still be too contaminated. (see comments). How about steaming food?
- Drinking: No
- Doing dishes. Yes, maybe. But you might want to rinse them with a bit of fresh water afterward. But it could still help
- Laundry: Maybe. Help needed answering this. Is salt bad for my clothes? Does salt effect the effectiveness of soap? (The comments suggest it does so in a negative way)
- Flushing the toilet. Maybe (see comments) The sewege system might not like the salt water.
- Washing my hands. Yes (?)
- Showering. Maybe. See --> Bathing
- Irigation (not pictured). No. Salt is not good for the land.
Reference: this picture (How much water do you use in liters) Source: https://www.vitens.nl/service/hoeveel-water-gebruiken-we-per-dag
Disclaimer: this is not a 100% complete answer. I hope it contributes by providing a list and some quantitative data on the current use of fresh water. I hope it will start a discussion, so we can fill in the maybe's The answers I've written after every application are only intuitive.
I think that for the most part of our daily use sea water can be used directly ( of course after proper filtering ).
Except for drinking why do we need fresh water, cannot saline water serve the same purpose. If there is a proper filtering set up ( just the same that we use for our city water supply ) then we can use sea water ( filtered ) for almost most of our daily use. Use a small desalination unit only for getting water for drinking and cooking. For all other use sea water can serve the same purpose.
It is more our mental thinking that sea water in not pure that prevents its use for general purpose.
Only precaution to be taken is to use PVC or other plastic for piping to prevent corrosion. Why cant even Municipal corporations use sea water ( filtered properly ) for even town supply using plastic pipes for transporting the water.
Some of the information contained in this post requires additional references. Please edit to add citations to reliable sources that support the assertions made here. Unsourced material may be disputed or deleted.