I think it would have been heard. Most likely by the mother ship to begin with. Sound travels very quickly underwater and it would be easy to time stamp it to match the time the sub was lost
Also, they were only 15 minutes from finishing their dive and navigating the entire way. I bet it won’t take long to actually locate them once they get assets on hand to search with ROV’s. I would suspect that they had a plan on where to approach the debris field and wreck from, and they’re not far from it.
The fact that it didn’t surface on its own with the tethers to the ballast suggests it’s either stuck in the mud, perhaps snagged on debris or the wreck itself or, and this is highly possible, Oceangate’s system failed or they pencil whipped the dissolving attachments.
I’d think w very wealthy families involved will foot the bill to find and recover them if the assets currently on hand fail or are called off. The thing will surface in its own if it’s still buoyant and an ROV can snip the ties to its ballast bags.
Lack of water is very valid. They surely had something onboard to drink. It’s they were going to be down for hours on end and probably had several bottles of water per person on hand or the equivalent. It’s not live on it for a month stuff, but maybe enough to last more than the standard three days.
It would be merciful if it did implode.