You should always carry water and food for yourself.
You should drink at least 2 litres of water in a normal day and more during periods of intense activity. If you can, carry extra water for other dehydrated people and for eye flushes if CS gas is used.
Preferably carry your water in cycling type water bottles (the ones that are squeezy with pop up tops) as these can be used for eye flushes (to treat CS gas).
Make sure you eat regularly as in adrenalin fuelled situations your blood sugar can get low, making you disorientated and a danger to yourself and others. Take food for yourself for the day. Also take emergency sugar rations eg. flapjacks, mars bars, kendal mint cake etc.