Chloral Hydrate is easily found online and makes it past USA customs just fine. Chloral Hydrate and GHB (or GBL/BDO) are two drugs which at higher doses, there is no way to resist the sedation. I say that as I have taken massive amounts of benzos before and wake up the next day with 100 regrettable texts and a new wardrobe of expesive clothes shoplifted from my local target instead of 12 hours of sleep.
CH tastes vile though so make sure to encapsulate it, also if the temperature is above 20 C or 50 F the CH will melt and deform the capsules within 1h making the capsules taste bad. So if you use CH encapsulate it only just before using it, and make sure to drink a full glass of water with it since it is a bit caustic on its own. Make sure to crush up the crystals (if it is crystallized) into a fine powder before encapsulating it though, so you can use less capsules. I think if you were to take over 3g (6-9 size 00 capsules) it would be close to impossible to stay awake. You can buy a supplement that comes in standard capsules or empty gelatin capsules online.
It was a prescription med for sleep until 2012, and although it is scheduled now in the USA, it is widely used for Microscopy in labs, so buying it from a certain well-known Chinese site that is on the clearnet and isn't just some drug or chemical site, they sell all types of things, is easy and 500g usually makes it past customs just fine. There area also other strong sedatives that you cannot resist the urge to sleep with, like Carisoprodol/Soma, Phenprobamate, Clomethiazole, and if benzos are mixed with any other sedative, even a decent amount of Diphenhydramine it will knock you tf out.
Also antipsychotics like Quetiapine, Olanazapine are impossible to stay awake with. Alternatively, Trazodone and Mirtazapine also have similar mechanisms and they are very hard to stay awake for. You could openly ask your doctor about Trazodone or Mirtazapine since they are not narcotics and technically antidepressants albeit atypical ones, just say your friend recommended them as he has been taking "X" and ever since then they haven't missed a night of sleep. Seroquel/Quetiapine is also prescribed for sleep often (in lower doses) despite being an antipsychotic and is arguably more potent that the other two for sedation, so you could use the same technique with your doctor to obtain a seroquel/quetiapine prescription.
All of these drugs make it impossible to stay awake in high doses. Whoever said it isn't as easy as the movies has def not tried enough sedatives. Benzos are barely sedating compared to any of this shit.
Nothing works like in the movies unfortunately. Your best bet would be to inject heroin. Failing that, take a whole load of benzos. A massive amount
The nausea from injecting heroin in an opioid-naive person would probably make it not exactly great for this. One could take a normal dose of an opioid to prevent nausea but cause notable sedation, and mix it with a benzo, some Diphenhydramine or Doxylamine, and wash it down with some alcohol.
But to prevent your body from awakening while drowning, a heroic dose of one of the drugs that I mentioned above would be more ideal, specially with high doses of combinations. I overdosed on Methylmethaqualone (a Quaalude anlogue) + my small dose of prescription Diazepam I was on at the time by taking about 500mg of MMQ, and EMT's did a sternal rub (basically extreme pressure on your sternum to induce severe pain and wake you up) and it didn't even wake me, despite fucking up my chest and ribs for a few months, I only awoke hours later in the hospital after being hooked up to a Saline drip and a tube in my throat to control my breathing.
So MMQ or Nitromethaqualone and also Ephinazone in high doses could be useful for this. They cause seizures yes, but you only wake up from them if you took a low/recreational dose. Otherwise all I remember is feeling euphoric, then sleepy, then woke up in a hospital with severe pain in my chest but I still felt euphoric.