that's a tough call as the ferry ride is quite nice. I guess it depends on whether you have lounge access in WLG. The lounge there is very good (I counted 14 types of beer a while back

). That would give you a bit of a break, perhaps a quick nap, definitely a nice shower, and some food while enjoying views of the ocean.
One thing to note is that the drive from WLG to NSN is not an easy one - it always takes me longer than I expect. Then again it's a beautiful drive. Can't you start your Tasman trip a day later and relax in Picton or somewhere in the Marlborough Sounds area, then take the scenic route to Nelson?