It can be very hot and humid, but isn't always. I've done five early October trips, and the weather has varied wildly from trip to trip. I've had:
Warm and pleasant, not too much humidity (upper 70s)
Hot and high humidity (low to upper 80s)
Super-hot (90s and above) with really, really high humidity
No rain
Brief afternoon rain showers some days
Some rain every afternoon
A hurricane! (Hurricane Wilma in 2005). That was an experience! But luckily, except for the couple of days that hurricane went through, the rest of the week had absolutely beautiful weather (see "warm and pleasant", above!).
So realize that it can be very hot and/or very rainy, and plan accordingly. But you might luck out and get great weather. It's a good idea to plan for the worst, and be prepared, than to assume it will be nice and be unpleasantly surprised.
Bring some rain ponchos and at least a couple of extra pairs of shoes for everyone (if they get soaked, you have another pair to wear until the wet ones dry out). Also, it's a good idea to bring hot-weather items like mister fans and "Frog Toggs chilly pads."
2 years ago early October was hot. Not a lot of rain. Every day was warm enough to swim. I would've preferred a little cooler weather, but it sure beat July!
We were there late September/early October two years ago and found temperatures to be in the high 80s with very high humidity, and only once did we have a brief shower during our 9 days there. As PPs mention, it can really vary but I would expect it to be relatively hot and humid.
how hot is the surface of the sun? I think its about the same temperature. Also it tends to have periods of monsoon level rainfall. Generally the extreme rains only last for an hour or two but occasionally it can be all day. We have gone in September or October every year for the past 5 or 6 years. I much prefer early November/late october to early october/late september
^^^ what PP said! As a native Floridian, that time of year trends towards approx 8,000 degrees. It can occasionally get a brief glimpse of slightly lower temps, but yeah, I'd plan for it to be pretty hot, though nights are usually much more pleasant than the brunt of summer.
The change in temps, with a glimpse of more fall-like weather typically happens in mid October. That can also be early or late October. But if you're planning on early October, I think the odds are pretty high it will feel like summer in Florida.