Let a bit of sunlight shine on a box of wood and steel (essentially that is what a barge is about) and an un pleasant heat builds up inside.
Set a heatwave on a barge and imagine what the interior is like: unbearably hot.
Here a few examples of how my neighbours deal with this.
This couple just avoid being inside and have hoisted a sail to create a shady spot.
Tent-like tarpaulins to dim the light that hits the steel.
No half measures here. Proud father of a one week old son keeps an eye on the active cooling system, while mother and baby stay inside - in a bearable temperature.