Why would frozen sausages defrost faster in water?

I do exactly the same. It is a very effective way of defrosting food fast.

Compared to air water has a much higher heat capacity and a much higher thermal conductivity. That means heat flows from the water into the sausages much faster than it would in air and the water cools less than air would as it heats the sausages.

Aluminium foil has a much, much higher thermal conductivity that either water or sausage meat. Provided the foil is tightly wrapped around the food, so there isn't a large air gap between the food and the foil, wrapping the sausages in aluminium foil will have almost no effect of the rate of thawing. Any small gaps are quickly filled with water as the frost on the surface of the food melts.

The heat transfer from 'hot' air or water to 'cold' sausages is roughly determined by Newton's law of cooling/heating:

$$\boxed{\frac{\text{d}Q}{\text{d}t}=hA[T_{\infty}-T(t)]}\tag{1}$$

where, for heating:

• $\frac{\text{d}Q}{\text{d}t}$ is the rate of heat transfer into of the body, which determines how quickly the body's temperature rises,

• $h$ is the heat transfer coefficient (assumed independent of T and averaged over the surface),

• $A$ is the heat transfer surface area,

• $T(t)$ is the temperature of the object's surface, as a function of time $t$,

• $T_{\infty}$ is the temperature of the environment; i.e. the temperature suitably far from the surface.

For now we'll assume that $T_{\infty}=\text{constant}$, throughout the heating process.

In that case, acc. $(1)$, the rate of heating depends strongly on $h$, the convection coefficient. It's well known that broadly speaking $h$ is much larger for liquids than for gases, all other things being equal. This table of $h$ values bears that out. So based on this it's reasonable to assume that defrosting sausages in water will be quicker than in air.

There is however a caveat. With water heating not only will the temperature of the sausages increase but also will the temperature of the water decrease. The latter, according to $(1)$, will decrease the heating rate. The trick to avoid this is to simply uses a large ratio of water to sausages or simply run the sausages under the cold faucet, continuously.

I believe it has to do with the speed of heat exchange between sausage and air in one case and sausage and water in the other. Also, frozen sausage essentially means that the water in this sausage is turned into ice - you can test that a block of ice in water melts faster than in air.

The most efficient technique is microwave in low power mode (if the power is set to high, it may cook the food prematurely), but many people don't like it since it seems slow.

Finally, another tip is simply to take the food out of the freezer 24 hours in advance and leave it in frigo.

I suppose my answer looks more like cooking advice than physics discussion :)