Mardet is a military abbreviation for marine detachment.
The mardet teams are likely marines trained in boarding any ship in the Fleet for crowd control or in the event of a takeover by forces internal or external to the Fleet.
Admiral Adama calls out a mardet alert team after the Hitei Kan, a tylium refinery ship in the Fleet, objects to the order to allow allied Cylons on their ship to make FTL modifications that could greatly extend its jump range (A Disquiet Follows My Soul).