My children play with the lego fishing boat (see picture) in our bath tube. This design does have a serious flaw (imo). Even if there are very small water waves in our bath tube, water enters the boat which then becomes to heavy and sinks. So if you are planning to built your own lego boat i would make sure the sides are high enough to prevent water from ...
The largest LEGO ship that officially floats in water is the Cargo Ship that comes in 7994-1 LEGO City Harbor:
It is the only set which contains the 74 x 18 x 7 Boat Hull, which is the largest boat hull LEGO has made.
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 (...
Yeah they float but make the hull wide not narrow as it will stay up right if you make the hull wide. Narrow means top heavy. I first started making LEGO boats now I make home built boats. And no need to be water tight for LEGO just get more surface area. Who knows you might even be able to make a cruise ship that floats.