New Lens Suggestions

Hi everyone. My name is Greg. I am fairly new to underwater photography using a dSLR but I love how the photos look when I luckily get a good shot. I have done a bit of research on my stock lens (AF-S DX Zoom-NIKKOR 18-55mm) and it sounds like it isn't the greatest for UW photography. This is my current setup below:

Nikon D3200
AF-S DX Zoom-NIKKOR 18-55mm
Ikelite housing
Ikelite 6 inch dome port
2 - Ikelite DS51 strobes

I am wondering what would be a good but not terribly expensive lens upgrade. I know the D3200 is limited in what lenses it can use. Most of my photos are close up but there are times I do take photos 2 - 6 ft away. I would like something that is AF and would fit with the 6 inch dome port. If this is not possible, what direction do you think I should go? Any help you can give would be great.

Thanks in advance.