Actually, depending on your diving locale, the water may be full of mind/body influencing chemicals. All sorts of studies have found waterside soils, water plants, fish and frogs with unusually high levels of estrogen, steroids, vasodilators, vasoconstrictors, methamphetamines, etc. I know that's not your question, but it is real, especially during the storm seasons when basically everything gets washed downstream to the bays, lakes and wetlands, remaining until either settlement or decomposition.
Sorry, can't help on the meds effect on freediving, I stick with organic herbs (used rarely).