The Watamu Marine Park may not be Belize or Australia, but it's considered by many to be the gem of the East African coastline. What does this mean? Well, basically it means that this reserve boasts some excellent diving/snorkeling opportunities in a beautiful and protected environment off limits to fishermen. Yes, it's located just over 100 km north of Mombasa, but don't let that small drive deter you. With pristine white beaches, hundreds of varieties of exotic fish, myriad colorful coral and other enticing sea dwelling creatures, these are relatively warm waters (25 C/77 F) replete with memorable sights and experiences. Dive the main coral gardens and Turtle Bay or, if you're feeling a bit more adventurous, explore the marine life outside Watamu's fringe reef, which includes beautiful rays and the largest fish on planet Earth: whale sharks.
Should you feel like extending your stay, you can always check out the Malindi Marine Reserve to the north, though the diving here is generally considered to be lower quality. There's also the Arabuko Sokoke Forest to the west, which is one of the last remaining dry forest reserves on the east coast of Africa. It's a great destination if monkeys and endangered birds are high on your to-see list.