The original plan was to borrow Kookaburra and use it instead of Atlantis to look for Ares. Trevor no longer needs to search for Ares.
I'm thinking that if Trevor is to complete his circumnavigation, he'll complete it on Atlantis, the boat he started out on.
As far as Trev is concerned, he may own Kookaburra/Ares, but he considers it his mothers boat.
Trev may stay in Australia for a while, but at some point he may need to return to Florida to take care of some things. He does after all own and operate a business. Atlantis won't be ready until April. But then again, he could always fly back.