To send a share map with a layer(s) turned off you simply go to the legend in edit mode and turn the eye off next to the layer you want turned off. This will allow you to still send that layer in the map but not have it turned on when the share map is opened. A user can then turn that layer on in the legend of the share map if they want to view it.
Tip: You can send the floodplain layer with your map but have it turned off initially in the legend.
See video below for a visual example.