Semporna is not a pretty town in the usual sense. It is a working port on the east coast of Sabah, busy with dive shops, seafood stalls, boatmen, market noise, and travellers trying to get out to the islands as early as possible. Its reason to exist for visitors is the sea.
That sea is enough. From Semporna, boats run to the limestone islands, coral reefs, Bajau Laut water villages, and dive sites that make this corner of Malaysian Borneo famous. Tun Sakaran Marine Park gives you lagoon views from Bohey Dulang, clear snorkelling water around Mantabuan and Sibuan, and a glimpse of communities that live much of their life above the water. Further south, Mabul and Sipadan pull divers from across the world.
Treat Semporna as a practical base, not a resort. Book permits and dives ahead, keep an eye on current security advice for eastern Sabah, and accept that the town itself is mostly a staging ground. If you have one spare day here, spend it on a boat.