This sounds like a great story! It’s great you know Florida Keys and this sounds like the perfect crime scene setting. Make sure you personalise your letter so the editor knows why they need to publish you.
Can I suggest a tight edit as Caron said? Try and take out redunbdancies and over-writing.
Eg New graduate detective Alice Delaney is charged with the investigation of a rape and murder in a close-knit commnity in tropical Florida Keys. Scarred by her own past university experiences of date rape, Alice throws out the rule book and sets out to solve this brutal homicide. But there’s something Alice doesn’t know. She’s being watched.
Ie take out things that are a little obvious eg, ‘every step of the way by the psychopath who committed it.’ Who else but the criminal psychopath will be watching her? (Assuming she doesn’t have a horde of disgruntled criminals tracking her every move or the crazy date rapists from uni days). The being watched part at the end also leaves me (potential ediotor) wanting to read more about what is going to happen. Then next PG I think you can detail more of the back story. BUT keep it short. I like the following PG. “Killer on the Conch depicts …. But take out ’frantic’ (I’ve assumed this already) and ‘troubled‘ (I also know this.) Short and sweet is the name of the game.
BTW I’m not trad publsihed, but I did do a 4 year diploma in professional writing. And one thing the course emphasised is: if you haven’t caught the attention of the editor in the first 50 words, they won’t read on.
I hope this has helped! I’m interested to hear how you get on.