Rob
Ansys Employee

Yes, pick fluid. It's a change on older versions as you didn't pick anything up until a couple of releases back. 

BOI is fine, adaption is another option. 

Lastly, the acceleration is because you're forcing water through a smaller gap: where else will it go in a 2d model? 

Fluent considers buoyancy for fluids, depending on density and if you turned gravity on. Solids would need moving mesh (or immersed boundary) and some other settings to be buoyant. The first example we had was a cube hitting water, then a lifeboat (getting off oil & gas platforms isn't that easy if it's on fire) and now things falling off aeroplanes