October is still the period of transition from dry to wet season. In November and December rains become more frequent, intense, and so more oppressive. Recent overall climate changes and global warming result in greater weather unpredictability, and in past years the shoulder period has widened. In 2014, for example, it remained relatively dry till December. Therefore, you might still get some nice sunny weather in Bali during the rainy season.
The peak of rains occurs in January and February. Monsoon winds get stronger, the sea becomes rougher, and the shores are often covered with dust and debris during that time of year. It doesn't seem very attractive, yet on the other hand, such weather assures thrilling raft, and also provides great swell, perfect for surfing, even though this seems to suit only the most desperate adrenaline seekers.
Like October, March is also considered the shoulder period between the two seasons, when rains become less profuse and less frequent, and land gradually dries up.