Marmo was ambushed and shot outside his home three years ago. Prosecutors say he was involved in a bitter custody battle with his ex-wife, Rebecca Braswell.
A third former Seabee, Seth Hardy, was sentenced to life with the possibility of parole for an earlier attempt on Marmo's life. A fourth defendant, Shannon Butler, was given 25 years to life as an accomplice to the murder.
All four worked at the U.S. Navy Construction Battalion Center in Port Hueneme.