I too carry the live aboard dream. People do it all the time, often for many years. I hope to as well, or at least for a few before I am too old to. I think it is a great spartan lifestyle, and once your boat is set up, can be very cheap.
As for SHTF, think of all the marinas there are on coastal waters? There are a lot of boats there, owned by rich Sheeple that will likely be abandoned as they never think of going out to open waters would be a good idea.
Why isn't it a good idea? well simple, because 99% of the boats are pleasure cruisers. They get stocked with booze, an afternoon's lunch and gass for the day, but little else.Even the sail boats are generally small, and not really meant to be lived on by thier owners. They are weekend toys, unstocked, and not set up for long term voyages.
Now, the minority of those who live aboard rarely, if ever leave the dock, because they are still teathered to society. Sure, maybe they move around a bit, and do a voyage or two here and there, but very few are set up to travel indefinitely. They are tied to land based jobs.
Now, if one were to set up their lives to live off of fishing, Anchoring out,and exploring, I am sure a boat would be the best BOL ever. Even Pirates would be of little concern,and here's why.
In the days of Piracy, there were well establish communities that traded with Spain, England and East Asian countries. In the Caribbean for example, there were well established trade routes, with regular shipments of goods and Gold going back and forth making easy, predictable targets for Pirates. After SHTF you won't have that.
What you will have are a small handful of live aboards who were living aboard prior to SHTF that are so far and few between that chances are you will never run into a hostile one. This is especially true because the sea provides in great quantity.
Piracy today depends on Fuel. It's done with small, short range, high speed boats. After SHTF where will they get the fuel? If there is no trade, and no fuel, there are no Pirates, becasue there is no easy, predictable way to profit from the activity. Going out on the empty waters every day will just be a waste of precious resources.
Also, lets say our Pirate freinds do manage to do this and find you. They are gong to be leary of a raid for one simple reason, they don't want to be shot out of the water and sunk either. A show of force will likely deter any further attack for a weaker target.
Another thing, a live aboard sail boat can sit darn near the horizon, well out of range and observe areas with telescopes normally used for start gazing and can then determine where it is safe to come close to land *IF* need be. One could cruise a safe distance, searching for speed boats, and only come in where none can be seen in range under the cover of darkness the next night. This would allow for some foraging,and light hunting. This would be especially profitable since areas that are safe would probably be the same remote shoreline areas all the game has been driven to from land based refugees.
A good sail boat has the full range of the ocean. It will be in the least populated areas there are on earth, and ultimately because of that, the least danger from raiders, marauder's and other common land based dangers.