I've shuffled through my pieces collection and found sadly none in dark blue, but all 17 I've found in either old grey or red have the same attributes you mention:
Top stud is hollow, with an H-shaped cutout
There is no LEGO logo or part number anywhere
There is a small seam that runs along the full length of the part on opposite sides
There is a small ...
My best guess, they are fake.
Mainly because of point 4 & 5
Lego is always very keen on hiding the seam caused by the molding
Lego include in most molds a mound number but they are always hidden out of view.
Can you read the number?
I don't have this version of the part but the other non-hollow one in white. I checked for these two marks and couldn't ...
It looks like the simpler problem would be to evaluate the boat hulls you could possibly find to see if any would displace 485 grams of water with enough free board to stay afloat.
Here is a list of all of the unique boat hulls I could find on bricklink that looked to me likely to hold water:
Boat Hull Unitary 16 x 8 Base
Boat Hull Unitary 22 x 8 x 2 1/3 (...
The floor and ceiling aren’t one piece. They’re made of four 30357: Black Plate, Round Corner 3 x 3 each with an assortment of other square/rectangular plates to fill in the gaps.
It looks like there are 20 studs between the corner pieces on each side. The second image you posted shows a layer of tiles holding the floor together. Presumably there is another ...