Mombasa Marine National Park and Marine Reserve protects the reef and lagoon off the north coast beaches, where glass-bottom boats and snorkelling trips head out over coral gardens and tropical fish. It is the easiest marine park experience from Mombasa, especially if you are staying around Nyali, Bamburi, or Shanzu.
Conditions matter. Visibility, tides, and boat quality can make the difference between a memorable reef outing and a routine tourist trip, so ask locally before booking and avoid operators that let people stand on coral. The reserve is still a worthwhile reminder that Mombasa's coast is more than hotel sand.