There are two ways of getting from Herceg Novi to Kotor.
Taking the Ferry
The faster and the more exciting way is by using the Kamenari-Lepetane Ferry.
View Herceg Novi to Kotor/Kotor to HN in a larger map
A one-way ticket for a passenger car costs 4.50 euros.
I love this ride! Here are some more pictures
Of course, if you are not in a rush and you want to explore more places, it is always better to take your time and take the longer route.
Driving from Herceg Novi to Kotor
View Kotor to Herceg Novi in a larger map
I suggest that you chose this option if you have never visited Perast. It is on your way and it really is a perfect place to take a break.