Would you recommend that I use my flash diffuser?
Soft light is usually better, I leave my diffusers on my strobes. Your strobe (if it's the one I'm thinking of) is going to be fairly low power, and with only one it might be hard to get good subject illumination for a large subject like a diver, since you need to be relatively far away from the subject to get it in the frame. This is where large dual strobes are more helpful. Small single strobes are OK for macro, but they may struggle to work on larger subjects, especially in relatively low vis water.