Luggage and bags:
Backpacks are the best as there are a lot of stops/transfers which require a lot of loading and unloading
Clothing/Shoes/Weather Gear: Light clothing, shorts, T-shirts and swimsuits
Toiletries and Medical Supplies: Pack up well as Tawau and Semporna are very small towns with limited shops and selection of products
Photo Equipment: Hand carry with you all the time. It's cheaper to buy film, batteries, memory cards etc. in Kuala Lumpur than in Sabah
Camera, underwater camera to be specific.
You don't want to be missing out on your prized shot, do you?