What benefits would be gained by using human laborers instead of drones in deep sea mining?
First let's address several assumptions in your question.
demand for resources only rises due to both a ever growing population
Not necessarily. The precious metals platinum and palladium are commonly used in catalytic converters in cars. Once you move to electric cars, demand will decrease.
scarce resources like platinum, silver, and other rare metals have been depleted on the surface
No. I refer you to this answer from Chemistry SE:
Will we ever run out of gold, silver, copper and other important conductors?
mine...the methane ice
Why? What do you need methane for? It's a nuisance. Even if you do need it, get it from landfills. Way easier.
Asteroid mining isn't considered a good alternative for earth due to the transportation of the goods back to earth. However if necessary the industry does exist (mainly for the initial space colonies)
Don't even get me started about this. Asteroid mining is a sci-fi fantasy which is nowhere even close to feasible or economical in the real world. I've commented on this on several answers and questions here in the past. If interested, finding these comments is left as an exercise for the reader.
The mines would be accessed by either mining ships or for the larger mines maglev trains connected to large underwater bases (with the tunnels dug under the ocean's surface)
and this one from one of the answers
Humans are smaller and can fit into tighter spaces and caves to detect the presence of rare minerals.
Deep sea mineralisation occurs on the sea bed or very close to it, within centimetres. There are no tunnels or holes to dig. Current deep sea exploration programs are targeting mineral deposits that are literally just sitting there waiting for us to pick them up.
Now, to why humans will be very bad for this:
- There is a reason why the deposits are called deep sea deposits. Because they are deep. Like, really deep. Four kilometres deep on average. Developing a submersible that can withhold the immense pressures encountered at these depths is expensive. While there were submersibles that reached deeper depths (up to 12 km), these were specially designed vehicles. Doing this on an industrial scale is simply too expensive. On the other hand, drones don't care about pressure (mostly).
One of the answers commented that:
You could also make humans generally better at identifying mineral veins or using complex tools than drones can (a specialist drone won't have all the tools required to do everything, especially if its meant to be good and cost effective at doing something).
This is not correct. If you've ever actually seen the deep sea deposits, they look pretty much like the mud around them. The human eye cannot distinguish the good stuff from gangue. On the other hand, drones can be equipped with instruments such as Raman spectrometers, pXRFs, IR wavelength spectrometers and a variety of other instruments that will be much better than humans in finding the stuff.
We can learn from today's mining industry. It is gradually becoming more automated with robots. And this is happening in subaerial mines, which are relatively simple to operate. There is absolutely no reason to introduce humans to extreme environments for mining, especially when we already have the technology to do this without humans.
Finally, if interested, the scientific-yet-not-too-technical magazine Elements published a series of articles on deep sea mining in their October 2018 issue. It is mostly paywalled (unfortunately) but shouldn't be too hard to find the full versions of the articles online. Alternatively, you can read the abstracts which are free. This is highly recommended reading allowing some understanding of deep sea mining as we understand it today, written by people who know what they're talking about.