Time
The 92km takes about 3 hours by a normal inter-island ferry from either Wellington on the North Island or Picton on the South Island. There are sometimes "fast" ferries during the peak season, but these have speed restrictions on them for most of the journey, through the Marlborough Sounds. The fast ferry would take around half an hour off the trip. You could also take a short plane trip from Nelson or Blenheim at around $90 one way.

Ferry prices.
There are many websites that sell Interisland ferry tickets. You must book during peak season, between December and the end of February. Booking during the winter is not really needed and there are plenty of sailings all year round (up to 5 a day during peak season, if weather permits). The Interisland ferry costs around $60 for a return pedestrian and around $185 for a person and car return (prices go up for campervans). Keep an eye out, as the Interislander line often offer specials. The only other major inter-island ferry is the Bluebridge ferry (aka Straits Shipping). They have slightly cheaper prices, but far less sailings.

The Journey
The trip can be a bit rough with the ships sometimes experiencing 7 metre swells. The ships are very sturdy and New Zealand safety standards are very high. If the conditions are deemed to be dangerous to passengers or cause damage to ships, sailings will be cancelled. If you have a tendency to get travel sick it would be advisable to take your travel sick pills or use whatever other techniques you use to prevent travel sickness.

Ferries between the North and South Islands are common and catching one is easy. Check in an hour before sailing if you are travelling without a vehicle, and 1 hour if you are travelling with a vehicle.

Take a look at both the Interislander and Bluebridge to see which company offers the best package for you.