Most likely, these are human traffickers, second most likely, they are gold traffickers, third most likely they are drug traffickers. Or maybe some of them are just fishermen, which is actually more likely than all the previous cases.
https://truthsocial.com/@realDonaldTrump/posts/1153966324414...
They are almost always used as narco trafficking. However, we only know that is usually the case, not that this particular one was used for that purpose.
There are a handful of civilian legal use submarines of this form and submersibles used for tourist and science purposes, however if a scientist or tourist rich enough to fund recreational submarines were killed it's likely the family would have come forward by now.