There is not clear purpose in sinking it deliberately with the woman inside in front of a lot of people and say that she was not here. Bones will remain and everybody knows where is the sunken sub. Lets suppose instead that there was (hypothetically) a lot of blood in the sub. To what extent would the evidence be erased in salt water? could you find blood remains in sea lice/other invertebrate's guts or in trapped water bags?
Going further in the idea, could the researchs analyze the biocenosis of animals trapped inside the sub in search of an anormal number of species of animals atracted by corpses? If there was blood or organic remains or a missing/hidden corpse a lot of scavengers will detect it from Km and came in a hurry.
How about electronic evidence? The previous locations of the sub for instance, indicating where a body may have been dumped. There are many kinds of evidence.
It would be circumstancial evidence only. Unless you find a body, traveling to the points X, Y and Z is not a proof of murder. And even if you find it, another person could have done this to incriminate this man or for unknown motives. We don't know if the journalist had enemies.
Of course such info would be really useful to start a searching but a dumped body can move with the sea currents so if is found in point X could have been dumped (or drowned accidentally) in the point W in fact.
Going further in the idea, could the researchs analyze the biocenosis of animals trapped inside the sub in search of an anormal number of species of animals atracted by corpses? If there was blood or organic remains or a missing/hidden corpse a lot of scavengers will detect it from Km and came in a hurry.