There are several places that offer great snorkelling opportunities along the Kenyan coast. Choose your time of year carefully though, to avoid the rainy seasons, which can adversely affect conditions and visibility.
My first ever experience of reef snorkelling was at Watamu; a small, laid back and relaxed village about two hours drive north of Mombasa. The snorkelling here is excellent. There is a nice hotel on the beach called Ocean Sports, or for a more Swahili feel, try Marijani Hotel in the village itself. It's less than two minutes walk from the beach.
Alternatively you could head south of Mombasa, to either Diani or Tiwi. Diani is a bit more developed, whereas Tiwi is very chilled out. It may be tricky to get to Tiwi Beach without your own transport, but there is snorkelling available off the beach there. There are various accommodation options in the area.
Near Tiwi and Diani there is also a Colobus monkey conservation project and a very large, community-run elephant sanctuary, which are also well worth a visit.
Snorkelling is also pretty good in some spots just north of Mombasa. There are some big hotels in this area, but they are generally quite expensive.
You could also try scuba diving while you're in Kenya, with it being one of the top-rated dive destinations in the world!
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