If my job title does not match my duties, should I put the title that matches on my resume or leave it as what I was hired as?

I’d leave the title as is and make sure the key duties or responsibilities you list focus on the security aspects. If you apply for a new job and they do employment verification, they would likely call the marina and verify the dates you worked there, job held, etc. If you say you worked security and the marina says “No, they were dock staff” your potential employer could interpret that as you lying on your resume and not make you an offer or rescind the offer. Not worth the risk.

That depends on your employment goals.  If you aren't seeking another security position, maybe you should keep dock staff as that can infer customer service.