Definitely doesn't look waterproof based on the welds near the top and the door, so I'd guess it's some sort of 'elevator' to deliver a diver in full dive gear from a ship to the seafloor.
I thought about that after you said it, but why would a fully-equipped diver want to be trapped in a thing like that on the way down to his dive? That door seal and latch would be halfway between leaking water and sealing ... ? It might as well be open.
Can't imagine what it really is .... Unless it's something like an Anderson shelter for someone who is NOT claustrophobic? I'd almost rather take my chances with a Nazi bomb than have to be inside that thing ....