I've never heard of a newer model auto-inflating due to heat, but older models would occasionally "go off" in a driving rainstorm or when a boat compartment leaks water. They've done a much better job with the new models and protecting the water sensor from accidental exposure.
The manual-inflate models are all right, but they won't do a danged thing if you become unconscious before/when you hit the water such as being thrown from the boat or if you passed out for whatever reason. So I'd get an auto-inflate vest if I were you, simply for safety sake because you never know how or when you're going into the drink!
Mustang Survival makes a very highly rated auto-inflate vest, but they are also one of the more expensive models to purchase. But that's what I have because I figure my life is worth spending a little more on!
Formerly Steve @ G & S