事故当時に航行を直接指揮していたコーエン哨戒長の行動が最大のポイントとなる。

。

The actions of Watch Officer Cohen, who was personally directing navigation at the time of the accident, become the biggest point.

Literal

Accident at-the-time [at-に] navigation [object-を] directly command-[was-doing-ていた] Cohen watch-officer [attributive-の] actions [subject-が] biggest [attributive-の] point [as-となる].

Formal investigative/news prose. 事故当時 ('at the time of the accident') is a set temporal phrase. 指揮する ('to command, to direct') is used for leadership in military, orchestral, or operational contexts. 哨戒長 ('watch officer, patrol chief') is a specific naval/military rank. ~となる is the formal/written equivalent of ~になる — 'become X.' 最大のポイント ('the biggest point') frames this as the central issue.