I don't believe there are any afloat opportunities for an 1815;however, you should contact the reserve IWC Officer Community Manager, Ms. Patty Pifer, at (901) 874-2976 or
patricia.pifer@navy.mil in order to get an official answer.
Also, just to make you aware, the DCO board is extremely competitive so if you call the OCM, take the time to also ask the question of what competitive looks like to determine what you can do to strengthen your application.