Hi 55diver,
Let me mention first that I run a dive business based in Kota Kinabalu, Sabah. I will share information on the best dive sites you could visit along the west coast of Sabah from KK - alongside Pulau Tiga & Mantanani. This will then help you to choose a Dive Operator who can best meet your needs for the dive sites you would like to visit.
There are a range of dive sites along the west coast of Sabah, all accessible from Kota Kinabalu. The ones you will go to depend up on the time of year you will be visiting Sabah and your diver certification / experience level.
Pulau Tiga (30nm south from KK) and Mantanani (~45nm north from KK) are both offshore dive locations. They are best dived from March - October (during the 'Dry Season'). During November - February we have the North East trade winds which impacts the sea state and can make offshore dive locations on the west coast of Sabah uncomfortable and not safe to travel to by boat. Both locations offer diving along shallow pretty coral reefs. Mantanani is renowned for it's stunning visibility and desert island like beaches. Pulau Tiga is best known for stunning corals and lots of fish life.
There are also three WW2 wrecks located 30nm north from KK in Usukan Bay - one is suitable for minimum PADI AOW Divers (or equivalent), the other two are suitable for technical diving only. This is due to the depth of the wrecks. The fish life, corals and macro life are stunning. We'll often see a large Grouper or Nurse Sharks at the Rice Bowl Wreck too.
Near the Usukan Bay WW2 Wrecks is Mayne Rock - pristine coral reef with two series of large boulders (20m - 5m) to go diving round and exploring. A nice complement to a wreck dive. For the wreck diving and at Mayne Rock you'll be the only divers in the water.
Last, but by no means least, the most popular dive destination from KK is Tunku Abdul Rahman Marine Park (TARP) - in terms of volume of divers who go there. Home to 5 islands and lots of different dive sites. All involve coral reefs (5m - 20m) and a wide variety of marine life. Perfect for chilled out leisure diving.
All Dive Operators in Kota Kinabalu offer TARP. Only a limited number of Dive Operators offer Pulau Tiga or Mantanani. Only one Dive Operaror goes to the wrecks & Mayne Rock.
Dive Operators in Sabah also have to be licensed Inbound Tour Operators - so the Dive Operator you choose should be able to suggest something for your dry day too.
Hope this helps you get the most from your diving in Sabah
DG