Is it advisable to dive/snorkel in that region of the world?
There is some recreational diving and snorkelling in Sierra Leone, but not much. There used to be a dive centre at Franco's just outside of Freetown, and a diving school called Salone Scuba, but both have closed down. As well as challenges related to the economic situation and low tourism, the water visibility is pretty bad (this is true for most of the West African coast), and the currents are complicated and strong - following local guidance is essential. That said, there are corals, tropical fish aplenty and wrecks from a number of historical eras.
What does still exist is a basic dive centre, Banana Divers based at Dalton's guest house, which also has a water activities centre including snorkelling. It's on Banana Island just off the southern tip of the western peninsula. It's run by a PADI-qualified scuba divemaster and keen spearfisher known locally as "Greg the Greek".
Wreck diving and spear fishing are probably the main attractions. Apparently there are wrecks near Banana Island that can be seen by snorkelling only, no scuba required.
Note that regarding scuba at Banana Divers, it's for qualified / experienced divers only. Greg used to do taster dives for beginners, but as of 2016 prefers not to. The equipment is in a condition that is usable for experienced divers who know which types of minor leak aren't cause for concern, but would be challenging for beginners.
Is there a serious threat of shark attacks in those waters?
The risk of sharks is even less of a consideration than anywhere else. The waters off West Africa are notorious for unregulated fishing, and the shark populations are dwindling. Barracuda and trigger fish are pretty common, but (unlike trigger fish in some other parts of the world) if you leave them alone, they'll leave you alone. Shark encounters are almost unheard-of.
The real risks:
- Rip currents still kill multiple foreigners a year - but mostly from the beaches, particularly night swims.
- Diving or snorkelling will usually be from a boat (either a small speedboat from Greg's place, or some people organise trips on rented local fishing boats), and so the biggest immediate risks are:
- Your boat captain falling asleep and not realising a current has taken you too far to swim back (a loud whistle is a good idea!)
- Getting hit by a fishing boat that isn't looking where it's going (make sure you're not somewhere boats go)
- A surprise current, undertow etc taking you into rocks or similar (get local advice on where to go and where not to go - and follow it!)
- For scuba diving specifically, be aware that the closest pressure chamber is the opposite side of the continent. Don't dive deep enough that decompression sickness is a risk unless you know exactly what you're doing.