The beaches of Sierra Leone are perhaps the most beautiful beaches in the world. In case you were around in the 1980s and remember the Bounty (chocolate bar) commercials, featuring a gorgeous woman on an even more gorgeous beach, these commercials were shot in Sierra Leone.
I don't think diving or snorkelling is a big thing in Sierra Leone, but it's certainly possible, though you'll have a hard time finding a local outfit to accommodate you.
I lived in Sierra Leone from 2010 - 2012. Shark attacks were never considered an issue. To the extent that I don't remember once anyone ever mentioning sharks while I lived in Sierra Leone. I lived on the coast and regularly went to the beaches.
More serious danger, though, comes from the treatorous currents at some of the beaches. Several foreigners drowned at several occasions while I lived in Sierra Leone.