Watamu has no direct bus service from Mombasa or Malindi. The standard approach is to take a matatu (shared minibus) to Malindi, then catch another matatu heading south and ask to be dropped at the Watamu junction on the B8 road. From the junction, a short matatu ride or tuk-tuk covers the few kilometres into Watamu village. The whole journey from Mombasa takes around two hours depending on connections.
From Malindi, the trip is much shorter — under 30 minutes. If you are coming from Nairobi, the easiest option is to fly into Malindi Airport, from which Watamu is a straightforward onward journey.