You can pay as you go for any drink. But whilst you can pay the difference price to upgrade any package, in terms of individual drinks you have to pay the full price. It’s about $9 for a Hendricks, no Botanist onboard last time I sailed
There is included sparkling water, it’s typically either a very low cost brand or water carbonated onboard. Both are inconsistent in terms of how “flat” they are to taste. We found it hard to tolerate what Onward produced it had had a bubbles bypass, it was far better on Journey. I know that will all be down to personal taste so it’s a try it and see on that one.