If you are in a relay, most people use the CamelBak or similar type of water bladder which is contained in a small back pack and has a tube with a one way valve and mouthpiece which comes to the front. The advantages of this system is that you can jump in to the boat very quickly if you are doing rapid changeovers and it stays with you if you capsize. The disadvantages are that they can chafe and rub and also that you are carrying extra weight on your back.