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. As well as challenges related to the economic situation and low tourism, the water visibility is usually pretty bad (this is true for most of the West African coast), and the currents are complicated and strong - following local guidance is essential, including when to go (around half-moon). That said, there are corals, tropical fish aplenty and wrecks from a number of historical eras.
There's a basic dive centre, Banana Divers based at Dalton's guest house, which also has a water activities centre including diving, snorkelling, fishing, free-diving and water-skiing. 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", who knows many interesting spots for diving and snorkeling around these islands. There used to also be a dive centre at Franco's just outside of Freetown, and a diving school called Salone Scuba, but both have closed down (though I've heard rumours that Franco's might start doing dives again some time soon).
One good thing about diving here is, everything's very shallow (most dives are 10-15 meters). This combined with the warm water (many don't even bother to wear wetsuits) means it's easy to have a good long dive, and on a good day for visibility much of the stuff (corals, fish, even some wrecks) is even visible by snorkelling. There are several wrecks that can be seen in the area, including some modern container ships that can be seen very easily, plus at least one 17th Century Dutch galleon off the north side of Banana Island, but difficult currents mean that one is for advanced divers only. There are also some spots that are sheltered by the island and have (by West African standards) excellent visibility, like 10 meters or so (which of course would be considered a bad day somewhere like Thailand, South Pacific or South Africa, but is easily enough for a comfortable dive or snorkel and to see plenty of things).
Banana Divers was briefly for qualified / experienced divers only, but in 2017 Greg has resumed doing "PADI discover" taster dives suitable for people who've never dived before. The equipment used to be a bit ropey, in a condition suitable mainly for experienced divers who know which types of minor leak aren't cause for concern, but he seems to have had it fixed. There are also several beginner-friendly spots with no currents.
Is there a serious threat of shark attacks in those waters?
The risk of sharks is even less of a consideration than other places. The deeper waters off West Africa are notorious for unregulated fishing, and the shark populations are dwindling. There's also a big shallow shelf that isn't ideal for sharks.
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 ill-judged night swims by revelers at beach parties. Be careful, don't swim alone, never swim while drunk, and if you do find yourself being swept out to sea, don't swim back into the current, stay calm, look out for rocks and swim to one side parallel to the shore, until you're out of the current, then swim back and take your time. Don't rush or tire yourself.
- 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 far (a loud whistle is a good idea! And at Greg's place they're very careful about avoiding things like this)
- Getting hit by a fishing boat that isn't looking where it's going (make sure you're not somewhere boats go - local knowledge is essential)
- A surprise current, undertow etc taking you into rocks or similar (again, 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. Not country, continent (and there are no direct flights either!). Don't dive deep enough that decompression sickness is a risk unless you know exactly what you're doing - but most of the diving here is very shallow anyway.