Your idea of building a dinghy/pram reminded me of this site I saw.
http://koti.kapsi.fi/hvartial/dinghy1/simboii.htm

He has a plan for building 2 very small ones out of 3 sheets of thin plywood. I have not tried one yet but they look fairly good.
If they work well I could see building one to keep and one for selling to pay for material costs.
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May set off to explore without any sense of direction or how to return.