I can't answer you question entirely but I do know that water consumption is seasonal. You need to drink more in summer than winter. I know this sounds obvious but many people abide by blanket rules that really are not useful eg "you should drink 8 glasses of water a day". Consider your biology. Your body needs more water when it sweats, when you are physical (exercising, doing lots of walking) when it is sick. Your body needs water to do almost every metabolic activity. Pregnancy speeds up your metabolism so you would need a bit more when pregnant when not pregnant.

Sometimes people get confused when they get oedema (puffiness that is related to water retention) they think that because they are retaining water they should cut down on their water intake... the exact opposite is true... if you are puffy then your body would benefit from drinking MORE water to re-establish a better balance. I know this is not your problem... you're not puffy are you? (ankles and fingers are good areas to check).

Check the colour of your wee. If it is dark then you need to drink more. Ideally it should be a very pale yellow.

Are you thirsty? if you are constantly thirsty it could be a sign of diabetes. Mention it to your doctor.

750ml is a reasonable amount to aim for but I'd try for more. I wouldn't say that is too much for a pregnant woman... a litre should be do-able for most people. (Eta: thanks Alan... I agree, more is better)

There is a good thread about water consumption called "The water Drinking Challenge" it's a sticky in the general health section.

If you consume caffeine in tea/coffee/cola then it will speed up your system and tend to make fluids pass through a lot more quickly.