Is this dangerous to cross?
As you can see on Wikipedia, fatalities are very rare, fewer than one every few years. https://www.passagedugois.com/crbst_3.html gives some tips to avoid sinking your car and some current information (tide, weather, etc.).
Keep in mind that the area is very flat: as a result, if the tide is going up and that you see some water coming and/or you see many more cars exiting the pass than entering it, double-check the tide times. If the weather is already on the road, unless you are extremely sure about yourself, don’t enter the road.
There are two possibilities to get to the island of Noirmoutier
You can also take a boat, e.g. Pornic-Noirmoutier, windsurf to it, or even swim if good swimmer. Not that far from mainland.