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Try it on land against something where sharpness is easy to judge - the lens SHOULD be very good.
I use it underwater with the ikelite flat port and extremely pleased with the images - fast focus and exceedingly sharp.  I generally shoot f/8 to f/11 (sometmes higher) and it is to my eye, perfect.
I always use centre single focus point to make sure the object i want is focus locked before recomposing.  With a strobe, shutter speed should not be an issue at all for the image provided the subject is well lit by it.

The ruler suggestion above on land is a good staring point to check the lens is actually performing as it should.

Underwater all i can suggest is that its focussing on the right subject and not auto picking something else in the image.

The extension ring on my setup is black and not transparent so its not light getting through there.

Ive got a canon 10-22 and 8" ikelite dome and have exactly the same problem - its a reflection of the front grooves of the lens and manufacturers name.
I get it when the sun is directly in the front OR when its JUST outside to the top left or right. This is quite restrictive given they're common shooting angles. It happens to me underwater as well, particularly shallow depths in tropical waters.
The first thing i did was colour in the lens writing with black marker and and that stopped those from showing. Someone recommended matt/rubber tape over the lens grooves to stop the reflection but i like the idea of the sock trick and will try that shortly.
The 6" dome does NOT have the same problem.