Yes, arborvitae should be watered daily when first planted, especially if they're large. After 2 weeks they should be watered every other day for a week, then every 3-4 days. Once they're well established (usually at 6 weeks) once a week watering will do. Please don't let them go 3 weeks without water! Mulch will help retain moisture in the soil, but a rubber mulch may heat up & dry it out.  Keep the mulch away from the trunk. When watering, think long and slow: turn the garden hose on very low (a few drops/second) and lay it down on the root ball for at least 1 hour until the soil is saturated. Once they're established let the top of the soil dry out before watering again.