With this warm weather and the regatta season in full swing now is the time for you to get your water system sorted. No use waiting to the last minute. Get used to using the real thing not a 600ml bottle unless fitted with a straw.

Not (so)  important for 7 – 10km races  ie less than 45-60 mins if load up a bit before (1/2 hour) the race  but any longer you need to drink  1 – 1 ^½ L

http://www.ausport.gov.au/ais/nutrition/factsheets/hydration2

Summary of Fluid Guidelines

• Begin each exercise session in fluid balance.  This requires drinking regularly throughout the day leading up to training or competition.  Have a drink with all meals and snacks.
• Immediately, before exercise commences, consume 200-600 ml of fluid.
• Develop a plan for fluid intake for all exercise sessions longer than 30 minutes. Aim to match previous fluid losses as closely as possible (within 1% of body mass). Take into account all the opportunities within the sport.
• Begin drinking early in the exercise session and continue to drink small amounts regularly.  Sports drinks or water are the best options.
• Replace any residual fluid deficit after exercise.  You will need to drink 150% of any fluid deficit in the 4-6 hours after exercise to account for ongoing sweat and urinary losses.  When fluid losses are high and/or rapid rehydration is required, sodium replacement may be required.  Sports drinks, oral rehydration solutions and salty foods can all contribute to sodium replacement.