Tap water is generally not potable. Liquids from sealed bottles nearly always are, and should be used wherever possible. Take care in restaurants as some may use unsafe tap water to make ice for drinks otherwise made with bottled/safe ingredients. Tap water in most hotels should not be used for drinking or brushing teeth unless explicitly labeled as safe.
Motorbikes are somewhat risky, but jeeps, trucks and other cars for rent are readily available at very reasonable rates, and are considerably safer. If you must use a motorbike always wear a helmet and never drive drunk the roads are hazardous with many large potholes. After a good time drinking in the party areas of Chaweng, Bophut, and Lamai, taxis are readily available to take you home. Be careful generally taxi drivers tend to rip off drunks. It's not rare for a tuk-tuk driver to charge you 50 baht for just a kilometre journey (for example, along the main Chaweng market) during evenings.
A recent development has been the appearance of quad bikes to rent for use on the roads. These do not appear to be registered for road use so take care.
Be careful on beaches at night time as crimes often occur there.
If travelling to Ko Samui, Ko Pha Ngan or Ko Tao by bus, do not store any valuables in your luggage. They have been reports of thefts from buses operated by "Songserm" Travel. Do not let your valuables out of your sight when transiting from the train to the islands.