The incident occurred about 43km (27 miles) from the city of Aktau in south-western Kazakhstan, during what the defence ministry described as a combat training mission. In a statement, officials said it happened "due to a sharp deterioration in meteorological conditions and increasing winds" as personnel were disembarking from a boat.
Detained are the chief of staff and first deputy commander of Kazakhstan's Naval Forces, two military unit commanders, and the commanders of a marine company and a boat. They are suspected of criminal negligence.
An interior ministry committee has been set up to investigate. Deputy Interior Minister Sanzhar Adilov confirmed a pretrial investigation into the deaths is under way.
Investigators are examining decisions made during the organisation and execution of the exercise, including whether safety requirements were observed, whether weather conditions were taken into account, and the technical specifications of the boat.
More than 500 military personnel, warships and coastal units were participating in the Caspian Defense 2026 exercise, according to the Kazakh Ministry of Defence.
The detentions suggest authorities are treating the deaths as preventable rather than an act of nature, though the investigation is still in its early stages.