Yes you are correct - and yes you are correct again. Get a gallon of drinking water in a store and use it for your watering. And it is almost in August - so soon the heat will subside - and you, your plant and the whole Long Island will be able to breath again. Right now your flowers are blasting - it can be too dry - or something else - but I think you guessed right. But since you have a large healthy plant - as soon as the heat subsides - you are going to have a full head of blossoms. Usually if the water is salty - like if you have a water softener in a house - the leaves are suffering too and the whole plant will wither and die. So hopefully it is mostly heat.

I use rainwater for my houseplants. I collect it mainly for my carnivorous plants. But remember, some AVs are stubborn bloomers and some frequent bloomers.