The main reason to come to Semporna is to get yourself over to the Semporna Archipelago, a short boat journey away. The dive companies are all conveniently located in the same area, and many have a dive centre at the resorts on Mabul Island. If you've booked your dive and stay from KK already, you'll be picked up from the airport by your respective tour company and spirited straight to Semporna's port to take you to your end destination, so there's no need to stay a night here.
You won't be using up your camera's memory card in the town of Semporna, which, except for its mosque, is not immediately captivating. There's a trash-strewn wet market and some pretty stilted, over-the-water hotels, but little reason to extend a stay beyond dumping your bags and chatting with dive-company staff – they'll soon have you salivating over the archipelago.