Although Nha Trang is famous among tourists for scuba diving, I did not dive here. What I did, and do recommend, is a boat tour. These boat tours are one day excursions. They can be booked in any hotel or travel agency in the center of Nha Trang. More or less they all offer the same. Boat, breakfast, snorkeling equipment, lunch, and an English speaking guide. Drinks are not included. Prices are very low.
If you book one of these island tours, a guide will pick you up at your hotel first thing in the morning. You will be brought to the local port. If you did not have time to have breakfast, you can buy something in one of the stalls in the port.