Hi Chris,
Welcome to the forum and congratulations on choosing to break free from the rat race.
Like you I intend to be living aboard one day. Have you ever spent holidays on board a narrowboat? It might be an idea to try it out (espeically in the middle of winter, emptying the toilet cassettes in the cold and the rain, etc!) to experience the worst it can be before completely changing your way of life.
As for self-sufficiency regarding food you may face some problems there. As Dor mentioned, you need specific angling licenses for specific stretches of water and there are fishing seasons, outside of which it's illegal to be fishing (because the fish need to spawn etc). Plus you might want to be aware of where you're fishing - canals are relatively static area of water meaning they're not always the cleanest of waters. Aside from any stagnation that might be in the water, pollutants, pesticides and fertislisers wash down from fields and industrial areas into the canal supplies, sprays from fields literally fall into neighbouring canals, and other boaters do (but shouldn't) expel detergents, oily trails and sometimes toilet waste into the water. It's not unheard of for dog-walkers to ping their doggy-doos into canals when "cleaning" up the towpath after their pet (Bleurgh!). Too many fish suppers from that "stew" and you're not gonna be feeling too great, mate!
Again without a hunting license and permission to hunt on an area of land (or indeed water) even snaffling a duck is illegal. (Even running over a pheasant in the road, it's illegal to pick it up and take it home! However the driver behind you can, bizarrely enough)
As for the lifestyle suiting a sufferer of depression - as you rightly say, that word means different things to different people. If it means needing medication or friends/family nearby for a sympathetic ear, you might not want to be cruising too far from them? You might not always have a decent mobile phone reception out on the cut to make a phonecall. A close friend of mine suffers from depression so it's rather an emotive subject. Yes, there's a sense of freedom with living aboard. But yes, there's still conforming to certain rules and regulations and the grafting to making ends meet, and license fees and fuel to pay for and shopping and laundry to do - so the change in liftsyle isn't necessarily about breaking free but fitting in to another mould. There's a great deal of appeal about it, but bear in mind that depression doesn't come from the external factors in a sufferer's life but is an internal coping mechanism. You'll still "cope" with adversity in the same way, it's just the triggers will no longer be the pile of paperwork in your intray and an arsey boss, but when the water pump breaks down at the same time as the stove goes wrong and you're in the middle of nowhere and wondering how you're going to cope with the -4 degrees nightimes...
Anyway, I'm one to talk as I'm not aboard myself yet. But I'm actively spending as much time as I can on boats, in as inhospitable times of the year as I can. And we've had no heating over Christmas while on board, and spent time with no hot water, and froze my hands off on the tiller in the December drizzle, pulled my shoulder operating a big ole b*stard of a lock gate but had to keep going, but despite these things I loved ithe life and still want to live aboard. (Although how well I'd have dealt with these things without my other half to share the trials and tribulations with, I don't know). Get as much experience as you can beforehand, is my advice, and ask questions to the the wise sages on this forum, because there's nothing these guys don't know.
Good luck with your dream, I hope it works out and I'll see The Endeavour Jean crusing past one day.
Lisa