The effects of cannabis can last for some time and are effected by dose, habituation, amount of fatty tissue and overall health.
The longer you've been using it, the faster the drug gets metabolized, but remember that it is also absorbed in fat stores in the body, which could prolong it's levels. It usually takes about 12-16 hours to clear. While typically producing a sedating effect, it may also produce a sense of anxiety in some folks ,known as a "dysphoric" reaction. It typically slows reflexes, hampers concentration and impairs thinking.
No human studies have been performed and it is unclear as to how hyperbaric environments impact upon it's interaction with tissues,(such as the brain), half life, (how long it remains in the body), etc. For example, insulin dependant diabetics, demonstrate increased sensitivity to insulin at depth due to changes in the tissue sensitivity. I could find only 1 animal study suggesting that at progressively deeper depths, there is a diminishing sedative effect, suggesting hyperbaric induced alterations in tissue sensitivity.
Behavioral Effects of Delta-9-Tetrahydrocannabinol Under Hyperbaric Air.
Medical research progress rept.,
NAVAL MEDICAL RESEARCH INST BETHESDA MD ; Walsh,J. Michael; Report Date : 1976
Abstract : As part of a continuing research program investigating drugs that may be used in hyperbaric conditions, the present study was undertaken to determine the behavioral effects of delta-9-tetrahydrocannabinol, the main psychoactive ingredient of marijuana. Operant techniques were used to establish sensitive baseline patterns of behavior in rats. Dose-response curves were determined for 0.5, 1.0, 2.0, and 4.0 mg/kg at 1, 3, 5, and 7 ATA with the animals breathing compressed air. As expected, dose-dependent decrements in performance were observed at 1-ATA conditions. Under the elevated pressure conditions, however, a reversal of behavioral toxicity occurred, where performance at 5 and 7 ATA was improved over that at surface with equivalent doses of the drug. These data suggest that the pressure reversals of anesthetics and analgesics that are reported at much greater pressures may well be detected at lower levels of pressure if more sensitive measures are utilized.
This implies that the effects of marijuana at depth are not as predictable as one would expect ; probably being a culmination of pressure, habituation, metabolism, physiognomy, dose and the inevitable "other factors."
Whatever your decision ; dive safely and responsibly.