Marines prefer to have low potential recruits.

Depends ocean or harbor. Ocean doesnt need marines but a sonar + PHH is useful. Harbor tile:
1 BO
2 Gunners
1-2 Marine +15 potential or Elite, BVE
the rest of your best Engineers

1-2 Marines is enough because adding more marines does not increase loadout speed/damage output. BVE does
Adding marines increases the number of marines on board.
Even with 4-6 Marines, it will take a ages to raze a harbor alone.
Therefore expect one or two other APA in the Fleet thus you dont need so many Marines.

Ok Thanks KingCong