A liner is a vessel designed primarily to carry passengers on a set schedule. There was a time when ocean liners were more than the mere pleasure cruisers of today. Before the advent of modern air travel, ships were the primary means of transport for those needing to cross the oceans. Untold numbers of immigrants came to this country in passenger liners, most in steerage class, a far cry from the modern floating pleasure palaces.
By well-lit, what I mean is situations that fall within the normal range of your camera's exposure program. This is a combination of ISO (film) speed, shutter speed, and aperture which will result in capturing a desired image. On land, such situations occur all day long, but underwater they are rare. Clear shallow water with a bright sun shining overhead is the only environment in which your camera is likely to work unaided, as it would topside.
An unremarkable ambient light test shot, at a depth of about 100 ft. Note the loss of colors, even close-up. Flash would bring out all the hidden colors.