It's unlikely that Kenya would be the first place many divers would think of for a diving holiday – a safari, yes – but this country is not regarded as a specific scuba diving destination.
The marine realm is interesting enough but limited by natural forces. The coast of Kenya is heavily tidal with fringing reefs that run right along the shoreline. These lie just below the surface and are exposed at low tide, which means that the outer reefs are not easily accessible and dives are mostly limited to mornings. Once you get into the water there is plenty of life. Hard corals are the basis of the reef structure with lots of small fish, nudibranchs and crustaceans. There are schooling pelagic fish if you dive on a day when there is a bit of current.
The reefs north of Mombasa seem to be in better condition than those in the south. Watamu Bay is regarded as the best area for diving as the bay is now a marine park. There are blue spotted rays and plenty of moray eels. If you're really, really lucky you might even see a whaleshark as it wends it's way up and down the coast. The operators promote this heavily but you are more likely to see a turtle and masses of nudibranchs.