About 10 km south of Kigoma, Jakobsen's Beach is the cleanest and most pleasant swimming spot within easy reach of town. A pair of small white-sand bays separated by a rocky headland, it is set in a quiet patch of forested shoreline that drops sharply into deep, very clear water. The owners run a simple campsite and a couple of bandas, so you can also stay overnight.
The snorkelling here is excellent for a freshwater lake — Tanganyika's endemic cichlid fish are extraordinarily colourful and thrive on the rocky drop-offs at either end of the beach. The water is generally safe (bilharzia is a risk in still bays elsewhere on the lake but seems to be absent here, where the water moves), though it is worth asking locally. Bring your own gear; very little is available to rent.