Ditto. Do not put on centre stand and leave in gear.
Some European operators do the tie down for you others expect you do it yourself. Personally I prefer to do it myself cos you'll always take more care of your paintwork than they do. It is normal to put one of the straps across the seat - use something to protect the seat - gloves are one option. Then put another strap to hold the bars secure. That's all it needs.
How big are the waves you are planning to ride?:-?
I rode the ferries in Washington State in July and 30+ bikes on each leg (Seattle to Victoria) and not a one tied down.
I'd probably put one across the seat and hold in place with first gear engaged and side stand.
Tie it just as you would on a trailer.
Connect straps to the handlebars near the riser. There will be D-Rings in the floor on either side of the bike. Tighten them 'till you've compressed the forks a bit.