Here's a basic breakdown of what you'll need to do to become a contingency (reserve) coxswain.
1. Be assigned to a STA with a RBS (defender class boat).
2. Become boat crew certified (4-10 months).
3. Pass the DWO exam (as of Jan 2010 NAVRULES will not satisfy the requirement)
4. Complete the PQS for coxswain (6-10 months).
5. Exhibit the judgment and maturity to be a coxswain.
6. Complete a oral board and underway checkride.
7. Maintain the currency requirements required for the certification.
Overall the process to become coxswain certified takes time and dedication. Reserve (contingency) coxswain certs typically take longer as they aren't immersed in the day-to-day underway activities at a unit. I know a few BM reservists who have been in for 5 years and still haven't achieved a coxn cert...