The Cryptologic Technician Maintenance (CTM) rating:

Cryptologic Technician Maintenance (CTM). CTMs perform a variety of duties associated with the maintenance and operations of cryptologic communications systems at sea with limited shore requirements. Apprentice and Journeyman CTMs typically provide deployed cryptologic support functions to surface and submarine combatants in either a PCS or TAD status. Due to lack of CTM
opportunities ashore, it is common for apprentice and journeymen CTMs to work out of rate while on shore duty. Master level CTMs have extremely limited sea duty and should be taking on challenging duties such as 3M Coordinators Afloat via Sea Special Programs when in-rate Staff Duty or EMO/AEMO opportunities are not available.

Source: Navy Learning and Development Roadmap.