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.
Free divers do but scuba divers do not need to. Scuba divers take their air with them and would have no need to hyperventilate.
they need to know how long they can be underwater, and how deep they can go with the air in the tank.
What they wear depends on what events they are competing in. Most athletes wear modern athletic attire that can include warm up suits, shorts and tanks.
for protection, as astronauts need to wear space suits to do space walks. Humans are fragile beings. Our skeletons and bone structure can only work in a narrow range of pressure around the atmospheric pressure [~1x105 Pa at sea level] -- we have adapted to earth's atmosphere. In deep seas, the human body will encounter a pressure [Pa], in addition to atmospheric pressure, equals to rho*g*h, where rho = water density [~ 1000 kg/m3]; g = 9.8 [m/s2]; and h is the water depth [m].
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.
Because fish have gills,giving them the ability to breath under water and divers dont.
We need air to breath. Otherwise we would suffocate and die. That is why scuba divers have special equipment to allow them to have air to breath underwater.
they need protection from stuff
Yes, astronauts wear specialized clothing while living and working inside the space station. They wear comfortable clothes for daily activities and special space suits for spacewalks or emergencies. These suits provide protection from extreme temperatures, radiation, and microgravity conditions.
They need to know how to swim away from sharks and such!
Well you need to be a member to get a special suit like Captain America, but I do believe that you can find the regular suits anywhere.