Buoyancy
The upward force that keeps objects afloat, equal to the weight of the fluid they displace — Archimedes' principle, applied to game water.
A boat floats because the water it shoves aside weighs more than the boat. Simulating that exactly needs costly fluid dynamics, so games cheat: a few probe points are scattered over the hull and tested against the water surface — often a sine or Gerstner wave, or a heightmap. Each point below the surface gets an upward force proportional to how deep it is submerged, plus drag that damps the motion. More points mean steadier bobbing but cost more. The trick carries everything from drivable boats and floating wreckage to drowning players, without simulating a single cell of fluid.