Visiting Sandakan, on the east coast of Sabah, in January will place you slap bang in the middle of the rainy season where unsettled weather may disrupt diving and travel plans, including thoughts of climbing Kinabalu.
The west coast at the start of the year is a far drier option with relatively settled conditions all the way through February, March and April.
Conversely the west coast of Sabah starts to have a higher rainfall from May until the end of the year whilst the east coast is relatively stable during June and July with afternoon showers helping to freshen things up a bit.
Be aware that July is also Sabah’s festival season so expect colourful processions as well as accommodation getting booked up months in advance.
Accommodation and flights are less expensive during the 'low season' from October - March and thanks to Sabah’s changeable weather there’s every chance of finding blue skies either side of an afternoon shower.
Although Sabah is certainly a year round destination, August and September are generally considered the best months to visit with the east coast getting wetter during October, November and December whilst areas around Kota Kinabalu, in the west, get drier.