Dome port for consumer video cam, is it worth it?

I have a little Sony DCR-HC36 consumer video camera. Most underwater video housing companies don’t even sell a dome port for those camera... a few companies do.

I want to use the wide angle on that camera… should I pay more for a dome port or it’s useless?

It depends on what sort of image you want. A dome port negates the distortion/aberration and refraction('magnification') of light off axis. It works for wider lenses but there are downsides in practical things like buoyancy etc.

Depending on your lens, you may also lose full zoom capability because of the virtual image created.