The main way to reach the Yasawas is aboard the Yasawa Flyer, a high-speed catamaran that departs Port Denarau Marina (near Nadi) daily at around 8:30am. The journey time varies enormously by destination: Kuata and Wayasewa at the southern end of the chain take about two hours, while the northern islands (Yasawa, Sawa-i-Lau) take five to six hours. The boat makes multiple stops along the way, so plan your itinerary carefully — missing your stop means either an extra leg or an overnight somewhere you hadn't planned.
Flyer passes are sold as unlimited hop-on-hop-off tickets valid for 15 days, which suits travellers who want to work their way up (or down) the chain and spend time at multiple islands. There are also fixed point-to-point tickets if you know exactly where you're going.
A faster but more expensive alternative is the seaplane service operated by Island Hoppers and Pacific Island Air from Nadi, which can reach even the northern islands in under 45 minutes. This makes sense for upmarket resorts or if your time is genuinely limited. Yasawa Island itself has a small airstrip served by these charters.