The phrase "tank dive" refers to a situation where a person or a group of people quickly swim underwater and explore the depths using a scuba tank for breathing. Full definition
Enjoy a 2 - tank dive in the morning and in the afternoon, check out the shore diving (weather permitting), relax at the pool or explore the island! (travel-masters.net)
They run scheduled single tank dives at 8:30 am, 10:30 am and 2:00 pm every day regardless of the number of divers going out and change the dive sites daily. (diveplanit.com)