It is important that underwater shooters understand that the strength of a given lens is not absolute. Let me explain:
The dioptric strength of a lens (+5, +8, +16, etc.) is relative to the medium that the lens' refractive surfaces contact (air, water, jello, etc.). So, while these FIT lenses have strengths of +5 and +8 in air, they appear to be water-contact lenses (correct me if I'm wrong!). If so, they are not +5 and +8 underwater, because typical glass achromats lose around 60-70% of their power underwater.
Timmoranuk - By direct measurement from the photos that Alex has provided, the magnification ratio achieved by the +5/+8 combo relative to the shot with no diopters is about 1.5:1. That's what you would expect from an underwater +5 diopter lens. This is consistent with the loss of strength due to water contact mentioned above. If this is true, then the FIT +8 is an underwater +3 (same power as the Inon UCL-330), and the FIT +5 is an underwater +2. A similarly constructed +16 would be an underwater +6 diopter lens (same power as the Inon UCL-165).
The MacroMate and SubSee are examples of sealed optics which retain their power both above and below water, since the lenses are always in contact with air.
All of this being said, Alex's results speak for themselves - they seem to produce sharp images free of chromatic aberration.
Just my two cents!
Nice one Keri. Thank you.