Drilling underwater is mostly to absorb the friction heat from the drill bit (either submerge the bottle or run a flow of water over the drilling area) but has some other benefits as well like not putting glass shards in your lungs. While a regular bit might work, even one designed to cut glass can very easily crack a glass bottle, even when submerged. You have to make a small hole and slowly drill out from there to expand it. Last time I bought a bit for glass it cost $3-5.