A float on Miracle - 20ft narrowboat

Cheap decathalon folder is ok, not amazing but then its now under a tonne!

Other cheap option is am old raliegh or bsa folder etc, could prob get an ok example for under 50 which us technically a better bike (and lighter) for about 50 to 75 quidi guess.
For storage put it on bank side walkway and lock to something solid (roof drain holes are good, or you could get a dedicated lock loop welded on at the workshop, you can use off the shelf crane/leggingfixings and about 30min to prep and weld

Or I still have an Airnimal for sale (pending Clefty having a look at it). It's got it's sea legs after living with me on my boat... :-)