Hi Hunter, welcome to the forum. In my youth and well before I had any book learnin, I embarked upon exactly this approach. My advice to you is to make sure you have a very clear and quick avenue of escape from the rapidly crushing and contorting drums at about 15 feet water depth. At every 33 feet of depth boundary, you are adding one atmosphere of pressure which from memory is about 14.62-lbs per square inch.
Putting that into context for you a 50-gallon drum is 23" diameter x 35" high. The surface area of that is around 3,359-square inches. Now with 14.62-lbs per square inch I will leave you to do the Math.
A short online search on Archimedes and his "Principle," should answer most of your questions regarding (or regardless of) the shape of additional components and their willingness to crush into almost unrecognizable shapes at relatively shallow depths.
A 50-gallon drum is designed to hold stuff in at atmospheric pressure and not to resist pressure to hold stuff out. If you are still hell bent on making a submarine, then the smaller the outer surface area the better. Make sure you have an up to date Will and maybe a Living Will in case they rescue you but not in too good a shape.