No strobe will work effectively at 20 feet distance. Effective strobe range is usually just a few feet, that is why wide angle capability is so important.
The strobe will reach out to 20' with just blue light getting back, but yeah it seems the usable range is just a few feet. I found it somewhat tricky to get the exposures right with the manual strobe. I would vary my distance and angle the strobe around. These pictures are my first underwater shots
Setup:
-Canon A720is with CHDK firmware shooting raw
-Intova ISS-2000 slave strobe with fiber optic sync cable
Overall I'm happy for the price, $130 with mounting arm. Recycle with nice batteries was 8seconds which seems like forever when you're fighting the current off Cozumel.
rest of album