As far as I'm aware Shellie any fluids you take, coffee, tea, water, counts towards your 3-4 litres a day but do try and drink more water.
A diuretic means that it actually dehydrates you (people who retain water are given diuretic tablets from the doctor for instance), that's the caffiene in the coffee that does that. Tea has less caffiene, maybe you could swap to that if it's a hot drink you're after.
LT advise drinking 3 litres a day of water but to be honest I can't even manage that amount and rarely make it over 2 litres plus the odd cup of tea. I think the thinking behind it is that the more you drink the less your body will retain it so your losses will be a bit more stable and constant, as well as obviously the water being good for your health!!
It's a hard habit to get into. Prior to LT I didn't drink any water at all, ever! Now I drink fizzy water all day and even through those 5 weeks I came off track I still continued drinking just fizzy water so I guess I have a developed a new, healthy, habit!
Good luck hon, try your best and have a chat with the pharmacist on Wednesday when you get your first weigh in if you're unsure about anything.
*hugs*