Is there an affordabe glass tube for a6500?

You have to remember that the cardinal rule of underwater photography is to get close, and if you think you're close enough, get closer still. With that in mind, 18-55mm has the potential to get considerably better shots, because 18mm will let you shoot the same subject from half the distance of 35mm - for example, a 50cm subject will fill the frame from 40cm at 18mm, and 80cm at 35mm. You can also use the 55mm end with an add-on wet diopter as a macro solution. There are, however, some nuances:

• SeaFrogs does not have a port and zoom gear solution for the 18-55mm lens; only for the newer 16-50mm. If you want to use 18-55mm, you will need a different, more expensive housing such as Nauticam.
• 18mm (or 16mm, for that matter) on APS-C is not generally considered sufficiently wide for underwater shots on its own; such lenses are often augmented by wet wide lenses like Inon UWL-H100, Kraken KRL-01 or Nauticam WWL-1 in order to extend their field of view to 100-130 degrees diagonal.
• On subjects that won't let you get close (e.g. sharks), the 35mm focal length might come into play, but any differences in IQ between lenses (already minute because you're likely to be shooting stopped down around f/8) will be lost in the noise introduced by the layer of water between lens and subject.

Regarding full-frame in general and Canon EOS R in particular, you need to keep in mind that underwater, full frame is often more trouble than it's worth. The light-gathering advantage doesn't come into play much because most of the time, you're shooting stopped-down with strobes, and the thin depth of field doesn't play well with dome ports, requiring huge, heavy, unwieldy and expensive domes. Likewise for macro - in most situations, you want as deep a DoF as you can get, which does not exactly play into full-frame's strengths. If you want 45-50MPx sensors for printing and cropping then you don't really have an alternative, but there are many people who shoot DX/APS-C, M43 and even 1" because of the problems with large sensors underwater.

Another point which is often ignored early on is that strobes, while expensive, provide possibly the best bang for your buck in terms of image quality improvement. Aside from the shallowest of depths, the best full-frame camera with the best lenses and the best ports using ambient light will lose miserably to a cheap P&S with a decent pair of strobes.