Can you see the Mekong and Phu Quoc in one day?

No. They are different directions out of the city.

Ho Chi Minh City to Phu Quoc

Fly. The ferry from the mainland exists and takes most of a day. On a short southern leg, the one-hour flight is the trip.

Mode	Time	Notes
Flight	About 1 hour	Frequent from Tan Son Nhat. Add the resort transfer on the island.
Bus and ferry	A full day	Only if you want the slow route via Rach Gia or Ha Tien. Not a connection to Can Tho the same morning.

Can you see the Mekong and Phu Quoc in one day?: No. They are different directions out of the city.