Nha Trang is flanked by 71 islands, and the sea at most times of year, is crystal clear so the plethora of boat trips daily heading out plus the easy availability of booking agents is self-explanatory. Trips are not expensive and are more often than not party cruises but with a fair amount of snorkeling and fishing fitted in between the drinking. Check whether lunch is included or not and compare deals before booking.
If you are not an extremely early riser then charter a boat the day before. Most hotels can handle the booking but the place to book in person is the Cau Da Dock.
