Skip the East Coast of Malaysia in November. It's Monsoon season, and the water (viz, surge) as well as the weather (wind, rain) can make diving less than pleasant. Why put up with that when you have better options all around? You can go to East Malaysia (Sipadan/Mabul/Kapalai, Mataking), of course. Or you can hop up to Phuket. Sipadan is a bit of a hassle to get to, taking a whole day to accomplish (flights, van ride, boat ride before you even get to your dive resort), but it is worth it. Phuket is easier and much, much quicker to get to (a one-hour flight from KLIA); and November is the beginning of the season, with excellent diving.
While in KL, take taxis. That being said, driving in Malaysia--if you stick to the cities and major highways--is not much of a problem. The main issue with driving in KL is that it is quite confusing to find your way from point A to point B. Malaysians drive slow (NOTHING like New York or Chicago), the traffic lanes are wide; but drivers tend to hug or even travel on the dividing line, so it's sort of hard to read their intentions. The biggest adjustment for an American is the prevalence of two-wheel vehicles in cities. There are thousands and thousands. Sometimes a hundred at a single traffic light at rush hour. These drivers often don't drive cars, so you can't count on them to pay attention to blind spots, stopping distances, etc. You won't need a car in KL, but if you want to go visit nearby places like Melakka, it's a great thing to have your own transportation. You can rent a car at the airport fairly cheaply.