OTOH the drug cartels probably have lots of money to invest. So if they need something sturdy they could probably hire a team to build that.

It's hard enough sometimes for normal companies to find the right resources. How many engineers are willing to work for cartels? Either they can do that by means of threat or fear, or finding the least ethically minded engineers that are looking for lots of money. I bet both cases are difficult to manage.

I think the cartels have already solved this problem in another area:

"Cartels have kidnapped technicians doing maintenance on cellular towers to make them fix their networks, people working in the sector said. The technicians usually are released after a few days, if not sooner."

https://www.reuters.com/article/us-mexico-telecoms-cartels-s...

Not to mention the collision risk of something lurking just under the surface with no lights or a radar reflector.

Yea because that's what drug cartels care about. Package loss is part of the process.