First of all....beer is wasted liquid! Alcohol is a diuretic (there is a biochemical series of reactions...but basically your kidneys go into overdrive)...so when you think you are drinking loads...you are making things worse or just drying yourself out!
Secondly...if you are thirsty, you are already a touch dehydrated
Thirdly, drinking too much is very dangerous (causing electrolyte imbalances by salts flush)
so, I go by the old addage, a good mountaineer always pees clear!
If I am going out next day and the weather is forecast hot I will make an effort to make sure I am well hydrated by drinking water during the day. I may (dont think this has any scientific background) make up a sachet of diorylite or similar and drink that too to ensure I am salts balanced.
On the day itself, I drink little and often (my bladder is a 3 litre one which I sometimes finish - I dont fill it to 3 litres, might half fill then refill). If I get very hot or am drinking more than normal (3 litres or more in the day) I often make up a diorylite in a small amount of water and drink that -but more often than not, I just wait till a convenient moment when back at the car or having tea and drink a diorylite then.