Canon SLRs and the Tokina 10-17 are a popular option, the tokina is the not the sharpest around but is reasonable and the range is good, though apart from very close focus wide angle work you need to change lenses to do semi-macro. The 18-55 will fill the frame horizontally with a 60mm long subject at closest focus, but the 18mm end is not that wide, similar to your TG-5 without an accessory lens. Bear in mind the 180° diagonal field of view is quite a different beast to your TG-5 even with the dome system and you need to get much much closer.
From the pics it looks like you freedive? you may want to physically hold the rig before deciding, the dimensions are 19x16x15 cm. It is quite boxy. The Pany LX-10 kit is much more compact.
The other thing to consider is a vacuum system, I believe Ikelite offer an optional kit. If you are popping up and down freediving, spending a lot of the time at the surface you are actually more prone to leaks - o-rings need to be loaded to seal properly and loading is minimal at the surface. The vacuum system pre loads the o-rings allows you to test for leaks before diving and also means it is physically difficult to impossible to dislodge the ports or housing back.
You have not mentioned strobes or lights so I assume this is not of interest? The ikelite requires electrical sync for strobes which is another set of o-rings to maintain - I much prefer fibre optic triggering.