You can buy liquor at the grocery stores on the island, but if you want a particular brand maybe you should bring a "starter kit" of i bottle[i bottle per person is legal entry].
I , and many others love the Day trip on the Winnie Estelle. 2 snorkles + a couple hours on Caulker. totally turnkey and great if you want to mingle with others. If you want more intimacy, contact any dive operater/ hotel helpers to arrange a more private boat; if you can get 1 or 2 other couples the price will be v reasonable, perhaps $150us for the day for the boat