It depends on how old your baby is.
You said he was premature... you should really be asking your Pediatrician.
For the first year of life, breastmilk/formula is the PRIMARY source of nutrition for a baby. If you give baby water, then don't make it his primary liquid source.
If you give too much water, you will be substituting this for the breastmilk/formula he should be having. They are also hydrated this way. Or, the water will make him too full to nurse/feed. You do NOT want to replace his breastmilk/formula with water, or use water instead of breastmilk/formula.
If you give them too much water, it will leech his body of essential salts/and nutrients. And then he will be too full for his feedings.
A baby is meant to have breastmilk/formula..... only until about 6 months old then water is "introduced." But even then, it does not become their only liquid or nutritional daily intake.
You best ask the Pediatrician...