DCL does not automatically charge gratuities. If you are on a CM or TA rate, I've been told that you are reqired to pay the standard gratuities at the time of booking, but you're sailing on a much reduced rate and you know that you have that little item already covered. Some TAs will offer packages with the tips "paid," but that is their/your choice and is usually done instead of an onboard credit that other TAs give as a "thanks for booking."
If you are a "normal" rate person like me, there is no way they ever automatically charge the gratuities. You must fill out and sign the form to have them charged. And, as above, the tips on a 3 night would be $36 per person....I have NO idea how you could get $304 for tips.
There is an auto-gratuity added to any bar item (this includes bottled water, candy in boxes from Preludes or outside a movie theater, smoothies, etc). You can choose to add extra to those tabs, but the 15% is automatic. Maybe you are adding all of those to whatever tips you authorized to get to $304.
I agree with the above--just say no. There is no reason to do bottled water on the ship. You could have saved big time by renting a car one way to drive to the port (instead of DCL bus) and stopping at a Walmart or grocery store for a case of water if DH wouldn't drink the ship's water (which is very good). $98 for a princess dress--we all like to indulge our children, but I look at the
Disney store on the day after Halloween, or usually made my own. Memory stick--probably would have been 1/2 the cost at Best Buy before sailing, or bring a laptop to download photos.