pH sensor, Salinity sensor and Dissolved Oxygen sensor.

Is it possible to make those sensors using Arduino?

What do you mean by this ?

If you have got a sensor from somewhere, it is almost certainly possible to use it with an arduino. The exception may be some sensors which use high voltage.

But can you "make" these sensors with an arduino ? Like "make your own" ? Probably not. The operating principles of these sensors are not simple and require materials and fabrication techniques that you probably don't have.