How is bottom time defined in scuba diving?

Bottom time in scuba diving refers to the total duration of time spent underwater during the dive, specifically starting when the diver's head enters the water and concluding when they begin their ascent toward the surface. This measurement is critical for divers as it impacts their planning regarding nitrogen absorption and potential decompression needs. The longer a diver stays at depth, the more important it becomes to monitor bottom time to avoid risks such as decompression sickness.

The other options are related to aspects of diving but do not capture the definition of bottom time. Duration of dive training refers to preparatory sessions rather than the actual dive. Time spent preparing for the dive pertains to pre-dive activities, which occur before engaging in scuba diving. The time at the surface after a dive is known as surface interval, which is relevant for dive planning but separate from the concept of bottom time itself. Thus, option C accurately encompasses what bottom time means in the context of scuba diving.