Water filters out more THC then it does tar . . . that is according to the only scientific study I've ever seen on the topic. Unfiltered smoke (pipes, joints) contained the most THC. So your likely not getting more THC per amount of weed burnt then other smoking methods,

Probably you are holding the hits in longer and/or taking bigger hits; since the smoke is cooler then most pipes would get it it is easier to "handle". In that sense you could possibly be getting more THC.

here's a link to the study
MAPS/CaNORML vaporizer and waterpipe studies