I am interested to measure the water depth in a tank above a reference point. The water depth or pressure varies above the reference point from 0 to 2.5 meters. How can this be done in a cheapest way without buying expensive piezometers and data loggers?

Without more data your question is essentially unanswerable.

Do you want remote electrical data or a big pointer on a stick which is read locally?

If remote do you want a continuous readout on a pc?
Is the pc local to the tank or miles away?

Expensive is different to different people. Water has weight and you can measure the depth of water with a pressure gauge/sensor.  In the same vein, a tube can be stuck in the water and air pressure can be applied to the tube.  The prssure remains constant at the point where air bubbles out of the bottom of the tube.  Great for liquids that are dirty.  Often people don't need what they think they need(Filter).

Cheapest way would be a suitably long length of wood with depth graduations in the unit of choice.

O/k being serious here. A pressure transducer is a good solution, but the cheaper ones will need some sort of amplifier to give a signal of usable amplitude. There may still be problems with long term accuracy and drift, and the results may not come up to expectations.

A very simple, low cost, and accurate depth measurement can be obtained with a float, a drum, and a ten turn potentiometer.

The required travel is 2.5 metres, the circumference of the drum would be 250mm, or around 80mm diameter. Don't laugh this is an entirely practical solution.

The typical linearity of multi turn potentiometers, even the very cheap ones is excellent, and if you wrap several turns around a grooved drum there will be no slippage. The voltage can be scaled to read directly in whatever engineering units are most suitable.

I have had great success with the plastic coated very fine, stranded stainless steel wire sold at fishing supply shops. This is used by fishermen to prevent fish from biting through the last few inches of line. This wire is light, flexible, incredibly strong and absolutely will not stretch.

Find someone that can turn up an suitable drum on a lathe with about a fifteen turn helical groove that will seat the wire. A light counterweight on the free end, and you are in business.

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