It's possible still today, but not as easy as it would seem. Most things you could do to a person's gear that they woudn't notice probably aren't going to cause enough of a failure of their gear to cause a drowning incident.
In 1969 a diver actually used far LESS gear than they do today so there would be less places to hide some sort of tampering.
On top of that we add that the classes were FAR more rigerous and strict in 1969 than they are today. Since scuba was still farily new as a recreational sport and the gear rugged at best, the people teaching it were using techniques similar to USN Frogman skills. What that means for your plot here is that it would be rare that the user of the equipment wouldn't perform some sort of thorough check of their equipment prior to diving. And there is really nothing a sabotuer could do to the equipment that would not be noticable at the surface prior to a dive that would fail suddenly during a dive.
The next issue you'd come up with again points toward training. Even today we teach people to make an ascent to the surface on a breath of air in the case of a complete failure of equipment. Admittedly most divers never ever practice it again after their basic class, but it is taught. Back in 1969 when gear was less reliable (often actually home made!) this was a skill that some considerable time was spent on during the class. So, even with a complete sudden failure, during that era of our sport, chances are very unlikely the end result would be a drowning.
One possibility could be Decompression Sickness (the bends). If it is a bad enough case one can die from it. Also, air expansion injury is another possibility. The air we breath is compressed but on the surface our bodies are not. Since we are mostly water, was we descend underwater, more pressure is exerted upon our bodies. Approximately every 33 feet we undergo pressure called an atmosphere. 33 feet the pressure on us is double what it is at the surface. 66 feet it is x3. 99 feet is is x4 and so on. If his ascent was from a depth of say 100 feet after having been underwater for more than say 20 minutes OR if he panic's and held his breath on that rapid ascent, there stands the possibility of an air embolism in either the lung or even brain (more rare), causing an extremely painful and very vivid expiration of the diver.
But your problem with making it believeable in 1969 is once again the level of training a diver received back then. Something would have to cause enough panic that the diver would forget their rigerous training and for what ever reason hold their breath on the rapid ascent.
So... here is the only potential possibility I can come up with. And even it's a stretch, but you are the creative one here, being a screen writer.
Divers cylinders are filled with air. Frequenlty you'll hear them mis-stated as being "oxygen tanks". This could not be any further from accurate and is one of the VERY FIRST things a real diver will notice and instantly know the writer hasn't a clue. We breath dry, compressed, filtered air. The exact same air you are currently breathing as you read this. Oxygen is actually a toxic gas under pressure or at higher concentrations. 100% oxygen is toxic to a diver in as little as 20 feet of depth. And it's a VERY nasty way to go. What's more, it's not a "possible" drowning, you wind up experiencing what's called o2 toxicity shock. The person experiences uncontrolled convulsions that are very painful and they cannot control keeping the regulator in their mouth. Basically they convulse, cough, choke and drown very painfully.
So, if a saboteur could gain access to the cylinders without the diver knowing and alter the gas, 100% oxygen would do the trick. They would not notice any taste difference when checking the equipment as they would with any other foreign substance, and it'd be a pretty definitive and graphic way to go as far as Hollywood is concerned.
Hope that helps!