Most people who have “hangups” about open water report that it gets tougher swimming around other people, not easier. I’m not saying it’s impossible to lose your fear of deep open water by jumping in with 500 other people, but it would be the first time I’ve ever heard of it happening.
An adrenaline shot that results in enhanced OW speed and a lack of fear is only one possible physical/mental outcome of waiting until race day. The other outcomes are much less desirable, and much more likely.
My $.02: find a couple very experienced friends to take you swimming in the ocean. Learn to navigate, learn to sight, and just try to get comfortable. Don’t worry about speed. Preferably, someone should be in a kayak keeping an eye on you. Do this several times before race day.