MIL-STD-248D has been replaced with NAVSEA S9074-AQ-GIB-010/248 for a very long time. Unless you are fabricating replacement parts for a ship that has been in the fleet for a decade or so, I doubt the old MIL-STD is the appropriate welding standard.
Assuming you should be welding to the newer version, i.e., NAVSEA S9074-AQ-GIB-010/248, there are a few items that have to be addressed. Paraphrasing

1) the welder has to complete a training program the covers all the fabrication standards involved, i.e., NAVSEA S9074-AR-GIB-010/278, MIL-STD-22D, MIL-STD-2035A, and any others that may be involved. (2) the welder must pass a written examination covering the fabrication standards. (3) pass a visual acuity examination on an annual basis.(4) pass the hands-on demonstration of skill.
The welder must not have any lapses in welding continuity, must pass the annual eye examination, complete the training and written examination every three years in order to maintain his qualifications and certifications.
Very few contracts that I know of still invoke the old MIL-STD-248D, so I would check the purchase order very closely to make sure you or your vendor is in compliance with the current requirements.
Best regards - Al