Onegaishimasu and Kudasai

Hi, what is the difference between onegaishimasu and kudasai, i know onegaishimasu is more polite than kudasai. Also, onegaishimasu is used when asking for service.

Then, why can kudasai be used in, "mizu wo kudasai"? Isn't that asking for service?

Also, is there any other diffrrence between the 2? Thanks in advance.

Both are polite, but:
onegashimasu is a request.
kudasai is an order.

Onegashimasu is used for requesting an item or object. It's also used when you're requesting a service or favor from someone else, or asking to speak to someone on the phone.

Kudasai is a polite way to order someone to do something. eg: 'Chotto matte kudasai' (please wait).

You can use 'kudasai' for an item too, but 'onegashimasu' is a more polite way and is usually what people say when requesting an object. In a restaurant, for example, you always say 'onegashimasu' when you're ordering food.

When you say 'mizu wo kudasai', you're saying 'water, please'. When you say 'mizu onegashimasu', you're saying 'please give me a water'. Both are acceptable but it's nicer to say 'onegashimasu'.