Is a cousin who died by drowning considered a martyr and among the people of Paradise, knowing that he did not pray but intended to do so before his death, and will he be tormented in the grave?

It is not possible to definitively determine the fate of your cousin—whether he is among the people of Paradise or not—or his current state—whether he is being tormented or not—because that is from the knowledge of the unseen. The creed of Ahl al-Sunnah wal-Jama'ah is not to bear witness to a specific individual being in Paradise or Hell, except for those about whom the Prophet, peace and blessings be upon him, bore witness. However, they hope for the righteous and fear for the evildoers. Abandoning prayer is among the greatest transgressions. Some scholars have held that it is an act of disbelief that expels one from the fold of Islam, while others have held that it does not constitute disbelief. Whoever dies by drowning among the Muslims is a martyr, as stated in the authentic hadith.