pH sensor, Salinity sensor and Dissolved Oxygen sensor.

Dissolved oxygen, pH Level, salinity, temperature, turbidity and micro servo motor using Ethernet shield data transfer attached all sensors in Arduino Mega 1280 ................ Is it possible to integrated all sensors in one Arduino board? And I'm also beginner on Arduino and programming. Please help!

Of course it's possible to create a board that contains all wanted items. But I don't know if this is a good idea, I'd leave the sensors near the medium to measure.

Definitely possible.
Mega is overkill for this, an Uno or derivative will do just fine.

Board is indeed best kept close to the sensors, especially pH and DO sensors need care to prevent noise, those and the EC sensor are prone to having problems with ground loops.