Update....2013.... Tahmeed now provide Bus service from Nairobi to Mombasa and possible to book Mombasa-Lamu combined. Nairobi Booking/Departure from River Road near the intersection of Accra Road...
Original Post*** Dec 2011
The cheapest (but not comfortable) way to get to Lamu Island is by Coach/Bus from Mombasa.
From Nairobi, various companies have frequent departures to Mombasa, I booked night bus with Modern Coast Express for 9.30 PM departure arrived Mombasa 6.30 AM next day. KS 1300 for Non Air conditioned Bus Business Class
Tawakal and Tahmeed Coach are most frequent and take 8 hours min from Mombasa to Mokowe vise versa as the road (well there is dirt track and can not be called Road) is bumpy with big size pot holes for 3 and a half hour - 100 km long strip from Witu to Mokowe. On my way back from Lamu I have seen road construction machinery and workers so I hope it will be paved atleast if not built mettaled road.
The Cost is KS 800 either way and it is advised to book a day or two advance for first departure and highly advised to get the seat in front to get less bumpy affects. Both companies have 3 departures daily each way.
At Mombasa, Tawakal and Tahmeed Offices (some 50 meters apart ) are at Abdel Nasir Road which is couple of kilometers away from the Nairobi-Mombasa bus terminal at Myembi Tyari Road, a Tuk tuk charge fixed KS 100 to get there.
Total Cost One Way Nairobi-Momsaba-Mokowe-Lamu was KS 1300 + 800 + 150 = 2250 including Bus/Coach and the Speed Boat.
There is flights from Malindi to Manda Island (Lamu) online booking is nearly USD150.00 one way but I have been told it is KS 3000~3500 if booked locally with travel agents.
There are frequent Speed / Slow Motor Boat / Sail Boat departure from Mokowe Jetty to Lamu.
Speed Boat is the best and fastest way (15~20 minutes) KS 150 one way, life jacket is provided for every passenger.
Slow Boats are KS 100 - 30 minutes (they depart in coordination with the Tawakal/Tahmeed - 1 hour before Bus departure time to report 30 minutes earlier at the Mokowe Jetty