Then allow me to ease your mind. Leeches are not a problem in the marine environment of the Florida Keys, unless you are a turtle. They person you replied to changed the topic slightly from the Everglades, where they could be a problem. In either case I'd worry about midges and mosquitos first.

Similarly with alligators, they are primarily freshwater and uncommon in the keys. American crocodiles can tolerate the marine environment better, but they are threatened as a species and have just two confirmed attacks in 75 years.

So wear a personal flotation device and you should be okay.

Fun story: I was slaloming bank to bank down that canal and wiped out. The canal is narrow enough the boat has to slow down and idle around the u-turn to then plane up to get back, so it takes a bit. There was a high arched water pipe over the canal and a kid parked on the apex. Kid sez, there's a gator next to you. I said, sure, right kid. Kid sez, there's a gator next to you... and I look and yep, maybe a 6', 7' gator about 10' away. Well... not much to do... I started waving the ski and a couple of minutes later they throw me the rope and I orientated and up and away I went. ha haha. Good times. I think I was 15.

Another one: Buddy of mine is on two skis and is kinda mellowing out just running down that same canal and I'm driving and see a gator ahead in the middle of the canal, and why not, I steer around the gator and then steer him right over it and it explodes in a huge splash ha aha haar I am just laughing at the memory and he looks back and then back at me with a big shit eating grin. I was probably 16.

Same canal: I got this hot gf I'm trying to teach to ski and she's fiddling with the skis, as you do starting out, and a nice 5' tarpon rolls about 6' away from her. Panic! We're like no no no they do not bite, it's just a tarpon, they're friendlies! Oh well, no water skiing for her. I was... 17.

But I'm not here to tell you these stories. I'm here to talk about the river of grass, the Everglades. Many millions have lived around the periphery but you can look at maps and see it's a long way across with "nothing" there. How would you see the vast scope of the interior, in an efficient way, right down at water level?

Family 2 doors over in Melaleuca Isles (still exists, I see) the father was the district superintendent (I think) for the Florida Fish & Game Commission, or whatever it's called these days. In those days the US was a normal country and everybody hung out, the kids, the parents. So I'm over there in the morning and he says want to go on patrol. I say sure. So we drive the airboat out to the launch point on 84 (Alligator Alley) and off we go. This thing had a Lycoming flat six and there's not much to the boat but the Al flat hull, the two tier seats, and the enormous engine and propeller. And for 5 hours, at speeds peaking at 100mph[1], we criss cross the entire sector of the Everglades north of Hghwy 84. I stopped counting deer in the sawgrass in the water at 100. The vistas were of an endless prairie of sawgrass. He drove across the hammocks where there was grass by just powering the boat onto the land and then over.

I came away from that experience with a full appreciation of the scope of the Everglades, the idea of it, and am sad that the idea of wilderness has softened like melting fat into an ideal of a cozy unthreatening warm bath. There is nothing that can be accurately described as wilderness unless organisms endemic there are present and may be out to eat you. Starting with mosquitos and ending with alligators.

[1] In those medieval times we did not know nor understand the term "eye protection" and so I had none, though my neighbor did. He didn't care. At 100mph your face is quite distorted. Some debris is getting through the screen on the front of the boat. What a MF adventure.