Can drinking water help to lose weight

Water is important to keep your body hydrated and all your organs functioning properly i.e. it can improve your digestive system. Some people also feel more full after drinking a glass of water, which is why a lot of people drink a glass of water before eating a meal to prevent overeating. But just drinking water without any change to your diet isn't going to do much more than make you need the toilet more often!
For the party, that's your choice. One night of drinking alcohol isn't going to have any long standing effects on your weight.

Drinking water doesn't directly cause weight loss. But it drinking more water could absolutely help you lose weight.

If drinking more water made it so you could poo without a laxative then you need to regularly drink more water. Pooing should occur naturally if at all possible. Some folks can't do it but you have found a simple and healthy solution for you. Drink enough and you poo enough.