Weather forecasting 50yr ago was more than good enough to tell you whether a particular day would be a good day (especially in the middle of North America where

Weather forecasting 50yr ago was more than good enough to tell you whether a particular day would be a good day (especially in the middle of North America where you can rely on observations further west and north to tell you about upcoming fronts). You don't need a week of clear weather to do this. Satellites gave us good long term forecasting (we know there is a hurricane that's gonna hit in a week or two). Computers gave us super accurate short term forecasting (we know exactly what time this afternoon it will start raining and how hard). Being able to know a day in advance what the weather will be has been a skill we have had with decent accuracy since the development of the telegraph (and the ability to know the weather in other locations much faster than the weather changes). Your primary concern in an un-seaworthy vessel (such as a bathtub) will be the surf. If you are not time constrained it is not hard to wait for a high pressure front to park its butt over the body of water you wish to traverse. It is only dangerous if you make it dangerous by being sloppy. Crossing the great lakes is an exersise in planning. Just because it requires basically no effort to do said planning today doesn't mean it wasn't very possible then. As stated in TFA people crossed lake michigan in all sorts of craft. The reason he chose a bathtub is because nobody has done it.