Crossing/skydiving the Cook Strait

i prefer the front of the boat OR outside.

Don't worry too much as it is generally quite ok. At least you aren't travelling on the vomit comet.

Outside is a good way to cure/avoid seasickness. But the crossing is rarely really bad. I also found that the Bluebridge ferry was remarkably stable the day I chose to cross during some sort of gale.

Greenlaces.
The strait is rough in a strong (or imminent) Southerly wind, only slightly choppy in a Northerly. (Vice versa for flying it.) The rough starts at the Wellington harbour entrance, and if Southerly, will continue to the Tory channel entrance on the other side, with really bad bits rounding Sinclair Head and Karori Rock, South coast Wellington area. About 90 minutes if they don't have to reduce speed.
The front of the boat is the worst place to be, lots of pitching, and consequently up/down motion, combined with rolling. A point near the middle or slightly aft of the middle of the boat (in length, and breadth) will result in the least movement. If outside, choose the sheltered side and focus mainly on distant objects, or the horizon. Inside, staring at a movie or reading a book is good, if you look outside you may find the perspective of watching the view moving up, down and around a bit disorientating. Drugs. Most pharmacies sell OTC seasick remedies for prevention. Root ginger has a reputation for settling the stomach.
The Bluebridge vessels are slightly smaller, but do seem to handle well, and don't seem to make the news so often after a particularly stormy time. The Interislander ones are larger, and perhaps smoother...I don't really know since I elected not to travel on them from the time coastal shipping was deregulated, the company was then sold to private enterprise, and I noticed that at least one of them is now registered in Panama. But back in the seventies and eighties, the Arahura was considered well built and appointed, and stable.
On a nice day the crossing is fantastic.