The legalities and logistics of operating a brewery vary state by state. Below are quick link resources for NYS craft brewery owners and operators.
Beer for ABC purposes doesn’t need to be grain based, sugar is the malt substitute in this case.
“Beer” means and includes any fermented beverages of any name or description manufactured from malt, wholly or in part, or from and substitute therefor.
According to the SLA, on the Standard Malt Beverage/Beer Brand Label Registration Application, for Question 17, Product Type, for hard seltzers you would pick the “Other” option.