Stick to drinking bottled water in Sierra Leone and try to avoid the sachets of water sold on the street – these are often contaminated.
The riptides in Sierra Leone's waters can be very strong, and beaches are not as a rule manned by lifeguards. Water might be contaminated by waste and fresh water might contain Bilharzia. Other than malaria, insect-borne diseases include dengue fever and zika. Consider pre-exposure vaccination if you're likely to come into close contact with animals.
While there are hospitals in major towns such as Bo, Makeni and Kenema, and small clinics in more rural areas, the best medical treatment is available in Freetown, where there is a good hospital and several private clinics. Most private clinics will charge a minimum US$50 per visit, not including any test or appointment fees.