All the following can be said, with different meanings and depending on the context. 1. 저 물 주세요 = Please give me water. (nothing implied) 2. 저 물도 주세요 = Give me water too (implies the speaker already asked for or received something else already) 3. 저도 물 주세요 = Please get water to me too (implies there's someone who asked for or received water) 4. 저는 물도 주세요 = As for me, please give me water too (implies there's someone else who did not ask for water) 5. 저도 물도 주세요 = Also get water to me too (Someone else already asked for water in addition to something else) 6. 저는 물은 주세요 = Please just get water for me (There is someone who didn't want anything) These diverse combinations mean you cannot draw up a simple rule as you asked about. You'll need to have a firm grasp on the individual particles to understand the different nuances and to know what should be used when. You can concentrate on the simpler cases first, like 1 through 3.

neither is correct. 저도 물 주세요 is correct.