MCDs may do beach recce stuff from boats on occasion but attack diving from boats tends to be more SF. Maintenance ops would be conducted alongside in peacetime so there's no requirement to carry MCDs routinely.
Besides, they get paid well enough without submarine pay.
You can't have divers and subamrines together. You would in essence have one bunch of people who think a shower per deployment is enough and are pasty white and fear the big yellow thing in the sky and the other lot who would, like absolutly die if they were denyed their basic human right to shower ten times a day, spray themselves in scent and practice their vollyball pose in the sun.
And they would implode trying to work out who has the most specialist pay.
You could have a submariner officer, qualified, who leaves the service to be an MCDO. Unlikely but possible.