Water is the most inexpensive beverage to serve....unless you are in a conference hotel. On an average, water bottles cost $6-7 (per 8 oz . bottle!) when tax and gratuity are added. This can come as a huge surprise to the conference planner when the final bill arrives.
It may sound silly, but you can negotiate a rate for the water in your contract (and if this event is in Arizona or Palm Springs this is a must). I've received funny looks from conference managers, and been told they'd never heard this request before, but they didn't refuse.
So how much can you save? We held an event in Phoenix for 750 attendees, a multi-day program, and negotiated a rate of $1/bottle plus gratuity - this saved $25k.
Additionally, you can request pitchers of water (free) in every meeting room, request these be refreshed often, and save bottled water for activities and boxed lunches.