Why do all my plants keep dying?

I've recently renovated my garden and bought some potted plants to brighten it up. I transferred them to larger pots on the day of purchase using Multi-purpose compost. I've been watering frequently (almost daily due to the recent heat) and had them only a couple of weeks but most of them have deteriorated rapidly in that time. I've included pics below. Any idea what I could have done wrong? They have direct sunlight ~4-6hrs a day.

The first pic is of the worst example. This plant has completely disintegrated. 2 weeks ago it was looking healthy with about 6 good flower heads.

The last pic is a flower still in good condition and even seems to be forming a new flower head. It's sitting alongside the others and has been treated exactly the same as the pink one which is losing its colour.

Any advice greatly appreciated. I have 3 cats but I haven't seem them interfere with the plants other than sniffing the pots when I first introduce them to the garden

 

First one looks like cosmos, think you might have overwatered?

Pic 2 looks a bit like the hardy version of Gerbera and appears to be doing ok.

Last two are hydrangeas which will eventually fade.
Probably greenhouse grown and acclimatising to the outside.

Both of those would be best planted in the ground for flowering in subsequent years.

 

Yes the first one is cosmos. It says on the tag to water daily and "keep well watered". I have watered only daily, although the soil was probably saturated each time.

The 2 plants in the 2nd photo were all glowing a few days ago. Now many stems are bent over and some of the heads have dropped all their petals and withered.

Interesting about hydrangeas acclimatizing; I didn't know that was a thing.

Thanks a lot

 

Yeah you do get info on labels saying things like, water daily, but its mostly rubbish.
You water when it needs it, so if its soggy you dont water.

Its one of the things lots of people get wrong, and often ask, how often should I water.
You water a plant just before it shows signs of drying out, ie. going limp.

If its soggy leave it alone.

 

Ok, that's sounds like a great life lesson. Annoyingly, when I pointed out that the plant was dying, my girlfriend immediately blamed me for watering it too much, in lieu of anyone else to blame. Seems like she was right. Thanks for the advice. I'll move toward reactionary watering now