Bigphillyfish,
You dont need flash if you have a good uw light that works for video too. i know many uw photographers that use just a uw flash light to take pictures with. i would suggest you go out and dive and take as many pictures as u can to see the results. for macro and close-up's this should work well. if you can afford the flash unit you will get better picture results.
i dont think the light canon is a good light for this. i use two eLed C8 but i also made a diffusor to "spread" the light beam. video lights are designed to have a wider beam, totally opposite of the light canon. i still use flash stobe units for my pictures and the C8's for my dive lights and for video.
really tho, just get out and experiment. you may be satisfied with the flash light only...you may not.