The nearest airport to Semporna, served by flights from KK and KL, is Tawau Airport, roughly 83km from town. A taxi from Tawau Airport to Semporna (1½ hours) costs RM100, while Semporna–Tawau buses (RM20, two hours) will stop at the airport if you ask the driver nicely. Buses that don't stop at the airport will let you off at Mile 28, a few (unshaded) kilometres from the terminal.
Remember that flying less than 24 hours after diving can cause serious health issues, even death.
The 'terminal' is vaguely around the Milimewa supermarket, not too far from the mosque. All buses run from early morning until 4pm (except to Kota Kinabalu) and leave when full. Buses to Kota Kinabalu (RM55 to RM75, nine hours), via Lahad Datu, leave at 7.30am and 7pm. Buses also go to Lahad Datu (RM30, 2½ hours), Sandakan (RM45, 5½ hours) and Tawau (RM25, two hours).
If you're arriving in Semporna by bus, leave the bus drop-off area and head towards the mosque's minaret. This is the way to the waterfront. Follow the grid of concrete streets to the right until you reach 'Semporna Seafront' – home to the diving outfitters, each stacked one next to the other in a competitive clump.