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How big is the root ball? You aren’t just watering the first couple inches of soil. Your goal is to slowly water the soil/mulch over the root ball so that the water does not run-off, and slowly percolates down into the soil, soaking that entire root zone.
The easiest and cheapest way to water shrubs is to lay a hose at the base of the plant, a few inches away. Turn the hose on a slow trickle and watch it. If you are getting a bunch of run-off, you need to turn the hose down. You will most likely need to have the hose run for about 10-20 minutes at 2-3 positions around the trunk. If that doesn’t seem like the method for you, you can use soaker hoses.
Avoid wetting the leaves, as they can become sunburned or even can be susceptible to disease.
https://www.gardeningknowhow.com/ornamental/shrubs/ninebark/growing-ninebark-shrubs.htm

It sounds like you had a caterpillar invasion. Something like gypsy moth, sawfly larvae and yellowneck caterpillars are a few possibilities. If the shrub was otherwise healthy and wasn't just planted, it will recover. However, a couple years in a row of defoliation poses a serious threat. Next year, start checking your plant a few weeks before this year's invasion happened. You may need insecticide if the numbers are too large to deal with individually. Post a picture here or at a bug ID website to confirm the pest then get a control agent that is labeled for that caterpillar.
https://extension.tennessee.edu/publications/Documents/SP609.pdf
http://ufdcimages.uflib.ufl.edu/IR/00/00/28/44/00001/IN62800.pdf

It may be a simple fertilization issue. This article will help in the meantime: https://www.gardeningknowhow.com/ornamental/shrubs/ninebark/growing-ninebark-shrubs.htm

My best guess would be that the soil pH is off. They will need a pH that is only very slightly acidic. More than likely the soil will be too alkaline, and will have to be corrected. This article will give you more information on the care of these: https://www.gardeningknowhow.com/ornamental/shrubs/ninebark/growing-ninebark-shrubs.htm

I'm wondering if it is a native ninebark shrub. Some have hydrangea like blooms. It can have round or oval leaves and sometimes with three to five lobes.

https://plants.ces.ncsu.edu/plants/all/physocarpus-opulifolius/

https://www.gardeningknowhow.com/ornamental/shrubs/ninebark/growing-ninebark-shrubs.htm

Summer is not a good time to plant things like this. It is likely having a really hard time trying to acclimate to the soil, out of season. Other than caring for it as it needs, there is nothing that you can really do to help it, other than providing temporary shade in the afternoon.

This article will offer information on their care: https://www.gardeningknowhow.com/ornamental/shrubs/ninebark/growing-ninebark-shrubs.htm