My skills and interests are completely different from my MOS. How can I fill in the Army gap on my resume to reflect that?

Never under estimate what you learned while in the Military, it has opened a few doors for me over the years. Yes you were a watercraft engineer, however while in the military what did you learn?... If you answer, you worked on combustion engines, I would slap the snot out of you. No you learned how to work as an individual, as a team member, you learned how to troubleshoot electrical and mechanical issues, deadlines, priories, self -respect. Pride in your work, your Company, and Country. Meanwhile why not look for work in a marina? Use those skills, even if it is part time. Maybe you were a squad leader,.. use that.

Me, when I have jeans with holes in the knees, some would throw them out,.. Me, I cut off them off and make short pants….
Good luck and never quit!!!

Employers generally will not care if there is a mismatch. I was Infantry as well before I seperated and went to college........my first job was at the very Top of General Motors a floor above the Executive Suite supporting the GM Economics Staff. Recruiter did not care one whit about what what I did in the Army but looked on the military service very favorably (it was EDS and that was the old Ross Perot influence, he was a Annapolis Grad and so was his Wife and EDS preferred military Veterans). So what I did on my resume was put Army as employer, years of service and a few bullet points of accomplishment while I was in the Infantry in the adminstrative area. So for example (going off memory here)....

Active Duty Army May 1984 - March 1987

Rose from PVT to Specialist Four in 15 months with promotional waiver, Worked independently and as a member of a team. Volunteered to take on role of assistant armorer while maintaining my other responsibilities. Worked with 1SG to create new Arms Room Policy for weapons maintenance and turn-in. Assisted with Brigade Arms Room Policy revisements. Earned Army Achievment Medal in performance of duties under difficult conditions while assigned as Gunner in exercise so and so. Honorably Discharged with Good Conduct Medal.

So something along those lines, I probably went a little long with the description but a little blurb under the Army to show that you were not some dud biding their time in the Army, that you actually have some accomplishments there. After you have more relevant job experience pertaining to the job your going for just drop the entire description under the Army and write it this way if you want to include it at all.

Active Duty Army - Honorably Discharged 1991..........something like that because after you have 3 other jobs it is all they care about. Depending on the job, since I have over 15 years experience now and the Army was the very start of my working career I sometimes just drop mentioning it. Federal related jobs I mention it or if I am applying to USAA I mention it, in those two respects it might have a beneficial impact. So that is how I handled it.

You will need to merge your skills from the Army into your future; leadership, dedication, work ethic, etc. These skills are valuable and were instilled in us as veterans. Apply them to your desire to learn Computer Science. Don't sell yourself short on your resume.