Comment by southernplaces7

Here are a few things to keep in mind in asking that question:

While spelunking, if you become lost or trapped, how long can you live without food (assuming your clothing is reasonably warm enough to protect you from hypothermia) while waiting for rescue? Quite a while, many days even. Most navegable caves have plentiful air and it being toxic isn't too common. Water is also usually present; it might not be clean water, but you won't easily die of dehydration in just a couple days at least.

Now imagine being trapped in a submerged cave, where none of the above applies at all, and you will die in a very specific range of seconds immediately after your extremely limited supply of tank air runs out.

Yep.

Also, silt lifting in submerged caves can reduce visibility down to a total zero in just seconds if you or a partner accidentally upset settled silt deposits with any sort of rapid movement. These can take more hours to clear than you have air to breathe, and in those situations, you'd better hope you have a guide line and absolutely do not let go of it at all.

Spelunking is very dangerous at its more extreme end (being the first to explore unmapped caves, going on multi-day trips into caves prone to flooding or other additional dangers, etc), but even normal cave diving makes it look like a sunday walk in the park.

I've seen many reports of utterly professional, extremely experienced cave divers dying during their descents despite doing everything they could think to do correctly. It can just be that dangerous. In some cases, this happens even in well-mapped underwater caves, and in a grotesque irony, there are many cases of them dying while working retreival operations for the bodies of other cave divers who just recently died in the same cave.