Semporna is the gateway to the most amazing diving in Malaysia. However it is also a dirty, smelly, poor place. Not as bad as small towns in Indonesia or Philippines but very close. The market is in the centre of town and you can see all sorts of produce. Fish is abundant as are many protected species. The bottled chilli sauce is excellent as are the fruit and veg.
The lower level town is dominated by dive operators and coffee shops. Food is good in a number of street cafes and you can even get western food at some locations. The upper town from the market has a wider range of shops and the petrol stations. The supermarkets have a wide range of goods and there is really no shortage of any basic supplies.
The bus and long distance taxi's go from the upper town just past May Bank and the Mosque.
Getting to the islands depends on your operator. Many resorts are now using the New Tourist Jetty which is 5min out of town past the bus station on the Hospital road. Other resorts still use the public jetty in the middle of lower town.