There are "limited" roots on each plant 7 along with transplant shock and not getting as much water as when they were at the nursery so YES water shortage is probably a major part of the problem.
trim back the bushes & start a regular watering program this year. a small amount of of stsrter fertilizer will also help
They plants require little treatment after they are establisshed.

Even though native and well-adapted to local growing conditions, newly planted shrubs need a period of time to become established before they can be assumed to be 'self-sufficient'. That would extend through at least one full growing season - often a longer period is required. "Some rain" is very likely not enough water if you have not been regularly irrigating them as well.

You can check to see if the ailing plants are truly dead by scraping off the bark on a lower stem or the trunks. If it shows green underneath, the plants are still alive and may benefit by pruning back hard to let them regrow (with adequate watering!!). I'd avoid using any fertilizer at this point - when plants are under stress for whatever reason, they are unable to metabolize fertilizers properly and applying it unnecessarily will only stress them further. If it is not green, then they are indeed dead and will need to be replaced. New plants will require regular deep watering during the period of establishment - do not rely simply on rainfall. It is not enough.

The other thing you should check is depth of planting. Many newly planted shrubs and trees fail because they have inadvertantly been planted too deeply.

What amount of sun do wax myrtles require? How much space between plants?
Thank you.