The sentence is not particularly well constructed. This would work better:
Whether or not visual observations from ships and airplanes, observations using underwater vehicles, manned or not, acoustics and finally trawling will provide the necessary detail to characterise litter and eventually define sources is not always clear.
Whether or not visual observations (e.g. from ships, aircraft or manned or unmanned submersibles), acoustic scanning technologies (such as sonar), or physical trawling (or dredging) will ever be capable of delivering the detailed data necessary to categorise specific pieces of litter (flotsam and jetsam) and trace them back to their sources is by no means clear.
Or better still:
Various technologies, including X,Y &Z are currently being used in the attempt to categorise specific pieces of litter (flotsam and jetsam) and trace them back to their sources. Whether any of these will ever prove successful, however, remains to be seen.