GHB is one of the fastest metabolized recreational drugs going. Its parent metabolite is gone from the blood within 4-8 hrs. The other big issue with it is that it's not part of a lot of routine tox tests. So unless you are looking for it or you send it for a more expensive analysis you will miss it. The other big issue with it is that people naturally produce GHB so trying to differentiate between natrual GHB or low levels/trace spiking and exogenous drug is a pain in the arse.
In relation to post mortem testing, if a body has been in a canal, you will struggle to get a reliable blood sample. This is true of any post mortem sampling but if your are relying on a cadaver that has been submerged in water, with bacteria, rodents and severe corpse decomposition. You will really struggle. You may possibly (if your lucky) get urine if the person has not emptied the bladder.