We camped on Lookout for 4 days, and drove up and down the island the whole time we weren't at our camp site. Just go to a car wash with the undercarriage spray afterward and you will be fine.
We've gone to Portsmouth a bunch of times and never did an undercoating. Just wash it when you're home.
I am still finding lots of sand in the upper reaches of the softtop and behind body panels from leaving the curtains off while the wind picked up to 30mph...so there's that.