Hello Fabio,
I am afraid your explanation is not very accurate. "Omicidio colposo" (Codice Penale, art. 589) is a crime committed with "colpa", as opposed to "dolo". The concept of "colpa" is given on art. 43, which establishes the "elemento psicologico del reato". Applied to "omicidio", and simplifying things quite a bit, it means the agent did not intend to kill but acted negligently and provoked someone's death. The corresponding crime in common law systems, to the best of my knowledge, is "involuntary manslaughter".
Chiunque cagiona per colpa [c.p. 43] la morte di una persona è punito con la reclusione da sei mesi a cinque anni. Roughly translates as:
Whoever causes by "colpa" wrongful act [As defined by Art 43] death of a person is jailed from 6 months to 5 years.
Art 43 for the part concerning wrongful act:
[The event/crime] Is not wanted by the agent and occurs due to negligence or imprudence or imperfection, or for non-compliance with laws, regulations, orders or disciplines
So we need to understand what is the violation of law, regulations, order or discipline that the prosecutors ascribes to the 5 other divers.
IMHO it will conclude with a decision not to proceed
Simply failing to give assistance to someone in need, not having caused the situation of risk, is the crime of "omissione di soccorso" (art. 593), with a higher minimum sentence if a death occurs ("(...) se ne deriva la morte, la pena è raddoppiata.").
There is the principle of specificity which applies here. A more specific crime has to be considered before a general one. So in mi view (and again I am not a lawyer) Omissione di soccorso (which includes the possibility of death) is more specific and should take precedence over Omicidio Colposo.
The big issue here is that to be omissione di soccorso, the agent has to have "sensorial contact" with the victim (see it or touch it, knowledge is not enough) therefore it might not meet the requirement to be omissione di soccorso since they only had knowledge but missed the view of the distressed victims. So this case might fall under the more general crime of Omicidio Colposo if there is any "law, regulations, order or discipline" that have been violated.
Cheers