I agree y'all did nothing wrong fishing the canals.
Their response was that if any body of waters are fed by a public source then that body of water is public waters. We can not let home owners scare us into thinking they own the waters. We need to fight back and keep public waters public.
There are some waters along the coast (very few mind you, and none that I know of around the Corpus Area) that are tidal but are not considered public waters. Typically it's a manmade cut connecting public waters to private land to create a back lake/marsh area. Some of the WMAs have areas that are off limits to all recreational activities as well. 99.9% of the time if you can float it you can fish it, but there are some situations where that's not true.
Not sure what you mean by fighting back. I would just move on and fish the other channels. If you're going to hold your ground and the owner is continuing to harass you then contact the local game warden and report the land owner for violation of the Sportsman's Right Act.