Matt,
My son has lived aboard in the SF bay area during his time in med school over the last 3 years so I have some relatively recent experience figuring all this out.
First thing is to see if the marina where you want to live or the boat is at will accept you living aboard on the size boat you want/can afford. Our experience is that most marinas in the SF bay area want at least a 35 foot boat for anyone living aboard. Now some folks use the "sneak aboard" method so that they can live on smaller boats and also avoid paying the extra live aboard fees. And some folks sell smaller boats as live aboards with the "assurance" that this is OK. Our experience is different. The marinas we've been at have card key entry and can track coming and going. Maybe you can get away with it, but be aware there are risks.
Marinas also often want a recent (within 2 years) marine surveyor's report and of course insurance. Some inspect the boat to make sure it is suitable for their marina. Some times if you buy a boat and the marina likes the PO, they grandfather you in with minimum hassle. Though I've not found that the minimum size is ever waived.
So I'd do some research and make sure you end up actually making this work. That said, my son has thoroughly enjoyed the experience. We ended up with a 1972 Coronado 35 MS (known widely as the Windabago) and it has been great for us. It is the perfect studio apartment and you can't beat the monthly cost -- around $500/month as a legit liveaboard. It also got the family into sailing and we've had some great trips. The high freeboard is a real treat making passage in high wind -- we call it our third sail.
But the old boat is great for family cruising and a solid bay boat.
In fact the old Coronado got us to buy a smaller boat for day sailing (an Ericson 27) off Craigslist and that boat is one of the joys of my life. The Ericson 27 would work as a liveaboard (a bit tight depending on whether you can fit in the v berth to sleep.) But again the issue is academic unless you have hard evidence that the marina will allow boats that size as a liveaboard.
Hope this helps. Let me know if I can be of any assistance.
Andy