In regards to the body metabolizing and excreting THC: if urination is increased as a result of drinking lots of fluids, does that mean that more THC is leaving the body? To rephrase the question, is metabolism required for urination? Or can you simply urinate as a body process without metabolizing toxins from your body?
This question asks about the urinary excretion of THC. Before answering the question I think you're getting at, I'll first note that cannabinoids (of which THC is one) are primarily metabolized by hepatic cytochromes rather than being excreted directly. This article is a classical pharmacokinetic paper on the topic if you’re able to access it; this one is a more recent general review accessible on Pubmed Central.
Cannabinoid metabolites are excreted both renally (via kidneys) and in the feces, so I’ll understand your question to refer to the renally excreted portion. When considering renal excretion of any substance we think in terms of glomerular filtration rate, which refers to the volume of blood filtered through the glomerular capillaries per unit time, often estimated as creatinine clearance. The question then boils down to:
Does excess hydration increase GFR?
And the basic answer is: no. The kidneys control both volume and osmolality more distally, i.e. after the event of glomerular filtration.1
In the scenario you posit – drinking a lot of dilute fluid – osmoreceptors in the hypothalamus will sense the decreased plasma osmolality and suppress ADH (antidiuretic hormone, a.k.a. arginine vasopressin), the hormone that acts on the distal nephron to control water reabsorption. If ADH is completely suppressed, the kidneys can produce up to 25 L/day of urine with an osmolality as low as 60 mOsm/kg.2,3 This huge volume of urine, however, reflects water homeostasis achieved at the distal tubule rather than increased glomerular filtration. It does not correlate with increased excretion of THC metabolites or anything else at the level of the glomerulus.
Notes and References
1. If this makes no sense to you, you may want to check out this basic introduction to renal physiology that is available online. The referenced textbook in #3 below is more comprehensive but requires a library or purchase for access.
2. "Normal" urine osm ~300-800 mOsm/kg; plasma Osm ~280-290 mOsm/kg.
3. Christopher Lote. (2012). Principles of Renal Physiology. Springer New York.
P.S. If the question you really wanted to ask was, "Will drinking lots of water make my tox screen negative faster?".... I'm not answering that question. For reasons largely unrelated to the discussion here (but see references in paragraph #1), I suggest assuming that THC metabolites will be forever detectable via tox screen. ;-)