I agree, and have explained the same thing to DM candidates. I think they were just stretching to find practical applications for students to demonstrate a knowledge of Archimedes' principle.. But while the lift bag calculations offer some interesting exercises, I could never understand their practical use. Over the years I've been involved in using both bags and barrels for moving heavy objects underwater ... a few years back I spent a whole winter building a small rock reef at one of our local sites ... and I've never once performed any calculations. In the real world, you just keep adding air until the object you want to move floats off the bottom. The real fun begins when you have to manage it as you change depths ...
The actual training calls for the students to lift something from the bottom under control. That does take some skill, but in reality most people just send it up and stay out of the way. What is missing in the training is what a DM might actually have to do--take a weight DOWN while under control. That takes a heck of a lot more skill than bringing something up under control--adding just the right amount of air a little at a time without sending it back up to the surface is not easy.
Two weeks ago I took a very heavy weight for a permanent ascent/descent line from the shore, down a slope, and over a ledge at about 40 feet. It is similar to what DMs often have to do in our area as they take a heavy weight for a platform anchor out to the training location and sink it gently so as not to create a silt storm.