Wet lenses are external lenses that are mounted outside the underwater housing.
The lens can be a macro or wide angle lens and in most cases, the thread of the lens is 67mm (M67) as that is pretty much the industry standard.
Also M52, M55 and M72 can be seen.
Quality and wet lenses
It is a common mistake that wet lenses are not of high quality. That is wrong. Some of the best underwater lenses are water contact lenses. (wet lenses)
i-Divesite has a range of good quality Macro and Wide angle lenses.
SubSee has 2 outstanding macro lenses.
And Nauticam has a fairly new and incredible series of both macro and wide angle lenses.