An object is accelerated on land, parallell to the surface of a tank of water. The object is then released onto the perfectly still water, making it slide on top of the water.
Is it possible to calculate some kind of substitute for friction between these two surfaces, or is the problem too complex?
There are two sources of drag when an object travels though a fluid like air or water.
The first is viscosity. To travel through a fluid, you must stir it. Fluid elements near the object travel near the speed of the object. Fluid elements farther away travel near the background speed of the fluid. When one Fluid element slides past another, there is friction (except in superfluids.) The object must exert a force on nearby fluid elements to make them overcome this friction. The fluid elements exert a reaction force back on the object, slowing the object. This force is called viscosity.
The second is that an object must push fluid out of the way in front of it, and fluid flows in behind to fill the space where the object was. Fluid must be accelerated to move out of the way, giving it kinetic energy. The object must exert a force on the fluid to accelerate it. There is are reaction force from the fluid on the object, slowing the object. This is called an inertial force.
Both forces are present. But in most motion, one is overwhelmingly bigger than the other. I show how to estimate the ratio in my answer to Shooting two projectiles at the same time with different. mass. The ratio is called the Reynold's Number.
In most everyday motion, the Reynold's number is large, indicating that inertial forces dominate. In that case, you typically ignore viscosity forces. This is especially true when the object is larger than an insect and moves faster than say 1 m/s.
In your scenario, the object is moving through air, which exerts drag because of inertial forces. It is also sliding over the water. It starts to dig into the water because of the downward gravity force. Water pushes back upward. There are different ways this can happen.
Sometimes the object skips like a stone. The reaction force is enough the push it entirely out of the water. It leaves expanding rings at places where it touches down and is relaunched.
Sometimes the object slides over the water like a water skier. In this case, the reaction force is big enough to keep the object from sinking. It pushes a furrow in the water.
Sometimes the object floats like a boat. Water pressure is greater than the weight of the object, and this hold the object up. The object leaves a wake.
In all three cases, the object pushes water out of the way, and the inertial reaction force slows the object down.
That sort of calculation is routine in digital modeling of water flow over the contours of a ship's hull during the design phase. Search on hull flow modeling.
