Marijuana is not physically addictive. It is however habit forming. Marijuana can cause cancer. It has the same levels of carcinogens as tobacco, however far lower quantities (of marijuana) are consumed, thus makig the risk exponentially lower, but not zero. The lung irritation is generally only in users that either practice 'holding it', a technique that contrary to popular myth does not significantly increase cannabinoid delivery, or users that smoke marijuana with a high CBD/CBN content.
The increased euphoria some 'holders' claim they feel is in reality mild oxygen deprevation.
Water pipes will reduce "tar" levels significantly both because major components of "tar" are water soluble and through mechanical action. THC is not significantly water soluble and consists of generally smaller particulates, reducing the effect of mechanical filtering in water pipes. Water pipes do however reduce the level of THC to some small degree.
As i stated before, the MRJ has found that marijuana causes a long term increase in IQ relative to the genral population. The study did not however test specific cognitive domains such as learning, memory, psychomotor coordination, or the ability to weave baskets underwater using only your toes. The FACT is that it increases your global IQ score. Whether it increases or decreases specific abilities cannot with any reliability be determined either way.