The mill could either be positioned midstream which required access by boat for ferrying the grain & then flour back to the waiting cart or near the bank where access by a boardwalk or gangplank was easier. Most important of all was the need for a steady controllable flow of current which could power the often huge waterwheels - many of which were 6 metres in width & diameter.
The boats two hulls needed very secure anchoring to ensure they did not break loose, often the method used was to insert heavy posts angled towards upstream and hammered deep into the river bottom on which to fix their chains, these often  became a hazard to other boats -particularly if the mill had moved to another location & there are many reports of severe damage to other boats upon running into an abandoned one and there were many disputes between boatmillers & owners of freight barges who didn't consider millers to be classed as proper boatmen & there was often intense rivalry.
Unfortunately, boatmills were frequently involved in accidents of one kind or another. They were never built with any means of proper navigation, no tiller, no keel & should they break loose then they would be swept away downstream turning sideways and backwards until they either ran aground or worse still struck a bridge. Fire was another hazard due to their construction frequently being made entirely of wood, Bearings overheating - a problem common to all types of mill could result in a major fire if the miller was not attentive & being surrounded by water seemed to be of little help.
Another major hazard in the colder parts of Europe was Winter ice breaking up and huge chunks would come hurtling down in the early Spring thaw & many mills were towed into Winter harbours where they were protected from the worst of the Winter flood.
A boat mill is  basically a floating watermill, once common in many parts of Europe but noticably absent from Great Britain although records show there were several attempts to establish them on the river Thames in London and the last one was removed in 1811 after several accidents with other shipping. Unfortunately there seems to be no surviving contemporary engravings to show  how they may have looked.
 On large rivers such as the Rhine, Danube,Elbe and their many tributaries boat mills were once a common sight,  often situated  near large towns or cities several would be in sight of each other in a line along the river bank. Millers had their own communities and were thus able to help each other with repairs & protection from storms.
The prefference for a boat mill rather than a land based mill had many advantages, although the initial cost was high for a boatmill - it still contained exactly the same mechanisms as a land based mill but it also included the boats it floated on, however it did not involve the construction of a leat or millpond and there was little likelihood of a large river drying up in summer & as the water levels did start to drop then the boatmill could be moved to a more suitable position. Also there were no extortianate ground rents to pay other than the local river tax to the landowner
Little now remains of the original boat mills, a few managed to keep going right up till the 1960's and quite possibly one or two may still be in operation .
Although some of the German boat mills were huge superbly made craft with three floors complete with family accommodation  they were more often than not quite crudely  made with a single floor  & parts constantly in water didn't last very long with the wheel itself expected to have main parts replaced within five years, the planked  hulls were often fixed together with iron clamps -like big staples - & the joints stuffed with moss which when wet swelled up & prevented leaks, a coating of tar also helped altho rainwater frequently found it's way inside the hull which resulted in the boat rotting from the inside.
Boats in any given area were almost always of the same type, the tradition for a certain size & construction going back many generations with little variation.
There are some original boat mills preserved in outdoor museums & there are some modern reconstructions open for visitors.
  I have to confess that although I have made several models of mills I was not aware of the existence of boat mills. therefore all of the information I have gleaned is from the wonderful book
'Boat Mills in Europe from eary medieval to modern times' by Daniela graf
The model is based on a typical small boat mill such as would have been found in Romania and Hungary on minor tributaries of the river Mures such as the Aries.
The size of the model does not permit a wide expanse of water as a more likely location for a boat mill
You can see a further set of model Windmills & Watermills 
If you would like to ask any questions about the model you can email me
You can see more pictures showing how this boat mill diorama was made
                A floating Watermill
   model made by Vic Newey, Worcestershire, England