There are several ferry services between St. Kitts and Nevis, all with byzantine schedules that are subject to abrupt change. Most companies make two or three daily trips. All the ferries take about 30 to 45 minutes and cost $8. You can get up-to-date information about all the options from a central number or a Web site (click on ferry schedule).
Sea-taxi service between the two islands is operated by Kenneth Samuel, Nevis Water Sports, Leeward Island Charters, and Austin Macleod of Pro-Divers for $20 one way in summer, $25 in winter; discounts can be negotiated for small groups. There's an additional EC$1 tax for port security, paid separately upon departure.
Ferry information (869/466-4636. www.leytonms.com). Kenneth's Dive Centre (869/465-2670). Leeward Island Charters (869/465-7474). Nevis Water Sports (869/469-9060). Pro-Divers (869/465-3223).