They make boat sets with little weighted keel pieces that will give them enough weight to sit nicely in the water; this might have some form of ballast inside it somewhere...
According to WP: "The density of surface seawater ranges from about 1020 to 1029 kg/m3"
According to WP: ABS is 1.060–1.080 g/cm3
Lego sinks in seawater.
If you saturate a beaker of water with salt then lets go here for some results because I can't be arsed to go back to school: https://en.wikipedia.org/wiki/Saline_water
Roughly 1.193 g/cm3 which is more dense than ABS so Lego will float in salt (NaCl) saturated water.
