pass into
"Penetrate" in this sentence means "pass through" as in "pass through a barrier", as the suit created a barrier through which no water could pass. It also has the meaning of "enter through an orifice", as there were no holes in the suit through which the water could enter.
Their suits-similar to divers.
Diver Suits
Women started wearing business suits as early as the 1930s. These early suits were skirt suits not pants suits that can be found today.
Deep sea divers wear diving suits in order to prevent their body by the harmful effects of maximum pressure at greater depth in seas and ocean.
the suits have to be stream line to move faster in the water
1987
Deep sea divers wear special suits to keep their body from getting too cold. They also wear these suits to keep from being crushed under the pressure.
Thermal protection - water at depth can be very cold.
Both astronauts and sea divers wear special sits because they travel into atmospheres that are strange to our bodies. Our bodies were not built to stand extremely deep waters and outer space. The suits protect them as they go about their business in those places.
Scientists wearing hazmat suits
Scuba divers do not wear heavy metal suits. Perhaps you are referring to the bulky looking Mark 5 dive gear with the large helmet and the breast plate that weighs 64 pounds. Suit weighs 25 lbs. Each shoe weighs 25 lbs. Oh, the weight belt the diver wears is 60 to 80 lbs. All this weight is needed to get the diver to sink from all the air inside the helmet.