I actually come from Mombasa and have done plenty of diving there.
The diving in Mombasa is what i would describe as "average." The water tempt is warm all year round so you will only ever need a shortie, in the summer the weather can get quite rough with large swells comming in from the Indian Ocean, which reduces the vis dramatically. The best time of year is the winter, around Dec, Jan, Feb, where you will have good vis and calm seas.
I personally would not go to Mombasa just for diving, as in my opinion it is not good eneaugh to justify the the expense of the trip. However, if you go for a weeks beach holiday in Monbasa and get some diving in, and then a couple of days safari "up-country" it is a great package and well worth it.
The best dive operator is Buccaneer Diving, it is run by a guy called Bruce Phillips who has alot of local dive experience, and is a Padi instructor examiner or course director (one of the two.) The operation is very proffesional, and about four years ago they sunk a big ship outside the reef for diving which is supposed to be superb. Buccanner Diving has operations in numerous hotels around Mombasa, on both Nyali Beach and Kenya / Bamburi Beach. I believe they also run trips looking for whale sharks which pass by every winter. Their main base is in Voyager Hotel on Nyali Beach.
If you need any more info you can email me at
robertsoper@weststartv.com and i will help in any way i can, do a search on the internet for Buccaneer Diving and you will see their website.
As a footnote there is also some good diving around the south coast "Dive The Crab" is the main operator there, i think they have put down a wreck or two also.