Once you have secured your location, you will need to decide on refreshments. This is really a matter of personal choice and is sometimes regulated by the host location. If you are in a hotel conference room, you may be required to order from their catering department. The costs for these services vary widely. Personally, I feel it is an unnecessary expense.
Water is just fine.
Your audience came for the information, not the cookies. Wherever you hold your seminar, there are some details worth listing here. Have a registration desk where people can sign in. Provide name tags, especially for large groups. The sign in table can double as a display table for your brochures, products and other promotional materials.