Agency fees are generally deducted at one time, and senior officers do not need agency fees. The monthly deduction is the management fee paid by the shipowner to the intermediary. Of course, it does not rule out that the intermediary will not deduct part of your salary. The key is to look at the credit of the intermediary. Wages are basically paid on the ship, and some are paid once your contract expires. It depends on what you say.
That's about it. Remember to find an intermediary with better credit to introduce a slightly better boat.
Good luck. I hope I can help you.