What is water displacement value for LEGO solid hulls?
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 (Set 6429)
Boat Hull Unitary 24 x 6 x 3 with Red and White Racing Stripes Pattern on Both Sides (Stickers) - Set 4002
Boat Hull Unitary 25 x 10 x 4 1/3 with 4010 and Police Pattern (Stickers) - Set 4010
Boat Hull Unitary 28 x 8 Base
Boat Hull Unitary 32 x 10 x 1 2/3, Base
Boat Hull Unitary 32 x 12 x 4, Base
Boat Hull Unitary 38 x 10 x 5 2/3
Boat Hull Unitary 41 x 12 x 5 with Light Bluish Gray Top
Boat Hull Unitary 48 x 6 x 5, Base (Catamaran Half)
Boat Hull Unitary 51 x 12 x 6 with Side Bulges, Base
Boat Hull Unitary 51 x 12 x 6, Base
Boat Hull Unitary 52 x 12 x 6 1/3
Boat Hull Unitary 52 x 16 x 5, Base
Boat Hull Unitary 74 x 18 x 7, Base
Duplo, Boat Hull 10 x 18 Bottom Section
Duplo, Boat Hull 12 x 24 Bottom Section
Duplo, Boat Hull 14 x 27 Bottom Section
Duplo, Boat Hull 6 x 16 with 4 x 4 Cargo Hold with Green Bottom
Duplo, Boat Hull 7 x 12 Bottom Section
Duplo, Boat Hull 8 x 15 Deck Studs with Red Bottom
Duplo, Boat Hull 8 x 17 with Police Pattern
Fabuland Boat Hull Large
There are over 200 variations of these parts with colors and prints. Have fun!