i don't think that drinking a lot of water will significantly reduce the lithium level in your blood. water acts like a cleanser in the system, removing toxins built up through metabolism and carrying them through the lymphatic system were the toxins are removed by the kidneys. the body exerts an aweful lot of resources to recapture the water in our systems.
all medications are poisons in that they are toxic to our systems. sometimes the results of this poisoning aren't too good, like when someone combined pain relievers and alcohol and sleep aids. other times the result is more positive, as when your body metabolises lithium and it increases the amount of neurotransmitters to produce a more stable aspect.
the other thing to remember is that everyone reacts differently to drugs. i take 900 mg of lithium/day. if i took as much as you are tking, i might well have a toxic reaction. but the amount that you are taking works for you.
i doubt if your dr will question your voracity. she/he may put you on a higher dose to increase the amount of he drug in your blood. hte dr may also do nohing, loking at you, your drugs, and y9our progress, the dr may well be satisfied to see you where you are, regardless of the blood test results.
hope this helps.
That light at the end of he tunnel? It's an on-coming train.