Thank You Wikipedia for this photo.
There are size restrictions in many of these places that would make a large building difficult to find a home for, and expensive as well. However, a small houseboat, no larger than an average boat would be fairly cheap to keep at a marina, and certainly far cheaper than buying a lot and erecting even the tiniest house. Many marinas offer water connections, TV, electric connections and sewage disposal.
If you go to the Netherlands, there are whole towns built on canals and sheltered waterways, including in some cities like Amsterdam. The barges in Amsterdam are much larger than anything we would be discussing here, and in many cases are truly built on boats. There are barges and arks as well, and these would be much more stable to live in. Consider a well designed building towed on canals in Europe.
It seems to me that in these days of tough economic conditions, that it would be easier than ever to find someone who was trying to get rid of watercraft. Construction on a boat would very likely be top heavy, so do not count on taking one of these out sport fishing in the Atlantic. Most could in reasonably calm weather be towed to new locations seasonally. It might be possible to tow for long distances, even along the inter-coastal waterway of the Atlantic, as long as reasonable weather and wave action were present.
Contact your local authorities for information on mooring in lakes for say the summer, your local marina may be able to offer advice on requirements in similar facilities.
An ark would be most likely to fit into our tiny house scenario. In this, a floating dock structure is constructed of treated timbers. The framework is filled with foam pallets made for floating structures.. Then a deck is constructed on top, of either water and salt(if on sea or brackish water) resistant woods like Teak(no small expense and a very heavy wood), and a house constructed with water resistant, or marine grade materials. There are plenty of materials out there that are virtually impervious to weather, which might not be a good idea or even attractive for a standard house, but would be fine for this application. Extruded PVC comes to mind.
One thing you have to remember with artificial materials like this is fading. Color is very hard to maintain in direct sunlight, so stick with white. Touches of color can be added on trim, outdoor grade fabrics or stained glass panels etc. You might explore mahogany, cypress and cedar as alternatives.
Think summer house in Cape Cod, downtown marinas in Boston, Miami or somewhere on The Gulf(if you dare).
Thanks to Wikipedia for a picture of an Amsterdam houseboat, though this would really qualify as an ark.
Make sure that you build small. Do something that can be lifted completely off the foundation so that the "raft" can be replaced periodically. Also, consider bolting rooms to each other so a larger house can be disassembled and pulled off the barge and reassembled on land or on a replaced or refurbished raft. No plasterboard! Wood panels, pickled, if you want light colors would be a better choice.
If salt spray is not a problem, you might consider large planters with flowering vines etc trained over pergolas, made of extruded steel stock, welded into a framework or build a lattice of water resistant woods. I'll bet a scarlet trumpet vine would do well in this environment, forming a leafy roof. Use plenty of Pearlite or foam peanuts mixed into the soil to lessen the weight of the soil.
A small ark would easily be dragged onto a flat bed trailer and used in the winter or put into storage as well.
In very tidal locations, one might have posts pounded into the mud. Several feet of steel pipe would then be placed on the bottom of your ark, and slipped over the posts. The sleeves and the ark would then ride up like pistons in high tide situations and lower down to rest on the posts when the tide is low. This would be good in places where you plan to be on stilts for much of the year, but still want to be close to the water level. You would not be inundated during high tides.