some small modern submarines are like upside-down aircraft - they go down by using wings with negative lift, same as an aircraft goes up, so if power fails, they will float. unmanned submarines are usually neutrally buoyant and use thruster pods - propellers - to move them in all directions, including up and down. traditional submarines control depth with buoyancy tanks full of air, same as a SCUBA diver controls depth by inflating a vest with compressed air. sometimes small submarines have emergency ballast weights that they can drop, to surface in the event of equipment failure.

controlling depth with compressed air is a bit tricky. the buoyancy tanks are not particularly strong, and are open to the water at the bottom. if the submarine sinks, the air in the tank is compressed and becomes less buoyant, so water enters the tank and the submarine sinks faster. the same happens in reverse if the submarine rises. so the depth must be carefully watched and controlled by hydroplanes and by adjusting the amount of air in the tank. SCUBA divers have the same problem.

they are hollow and full of air, so the average density is less than that of water. there are no holes below the waterline to let water in, and they are designed and ballasted so they don't tip over. same as floating a tin can in a bathtub.

Builders have built ships up to about 1500 feet in length ad this is generally considered the maximum that can be achieved at this time with the materials available to us.

The marie classification societies have put limitations o the deadweight capacity of ships which is in the order of 275,000 metric tons.

Going the other way a boat must be at least as long to be able to carry a person.

Eczema is a disqualifying condition for a seafarer. It depends o the severity of the condition and its effect that it has on the person. You will need to seek professional advise o your own personal condition prior to taking the required medical examination.