(I didn't get a notification that you'd posted, so apologies for the delayed reply!)
You will certainly 'get away with one' of these because at 24mm your angle of coverage is about 85degrees, and this FIT outputs a beam of 120degrees. So in terms of an even coverage of your frame you won't have any problems there. This is why your dive torch looked like a laser beam, a very narrow beam
At 2400lumens it really is very, very bright, but as I rarely dive outside the UK I can't say if it's bright enough for daytime use in, for example, the Red Sea. At night, no problem at all, it's awesome. I also use this as my primary dive-torch on night-dives; as the 1000lumen spot setting is narrow, it is a very intense beam indeed.
Don't forget that there are three light settings for each mode (wide/spot) which give 100%/50%/25% outputs and subsequent increases in battery life. Quoted burn times at 100% are 1hr for flood (2400lumen) and 2.5hr for spot (1000lumen) which are pretty accurate. In the UK I rarely use this torch at 100% because it's like turning your headlights on in fog!
I hope this helps - you certainly won't regret getting this light, but I can't comment on it against the I-Torch as I've never used one. I would recommend you get a spare battery too.