Are you talking about an external diopter or one that screws onto the lens? Since you're using Aquatica and likely have their ports then Aquatica makes the Closeup kit that's matched to their macro ports and comes in a +5 or +10 power. With external ones, you don't need an extension, just a mounting system, an internal one may need a short extension depending on the power.
Here's the link to the Aquatica ones. http://www.aquatica.ca/en/accessories_closeup.html and a link to the Subsee ones http://reefnet.ca/products/subsee/
External ones are more flexible as you can remove them and basically have 3 magnifications underwater. A screw on type close up filter that mounts inside the housing pretty much limits you for the whole dive.
You loose working distance with these lenses but gain a lot as both brands are multi-element and retain their magnification underwater, much more so than single elements ones.
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Philippine Dive Trip Specialist