Yes, you can often do this substitution and I've done it successfully many times when I've run out of milk unexpectedly, but you should know that the substitution is never going to be perfect.
On a purely mathematical basis, let's say your heavy cream is 36% M.F. You need partially-skimmed 2% milk for your recipe. If you go purely by weight (which is almost 1:1 for volume when talking about mostly water), then 250 mL of 2% milk will contain about 5 g of milk fat. To get that from 36% cream, you only need about 14 mL, or approximately 3 teaspoons of cream. (Note - don't do this, read on!)
Of course, diluting cream at a ratio of 15:1 is ridiculous and you'll end up with something that's just really watery, and that's because milk is more than just water and fat. I only provide the above calculation for illustrative purposes, to show why there's no hard rule or even rule of thumb for this particular substitution.
With normal cream, most people go with half water, half cream, or if they're trying to approximate skim milk then maybe 1 part cream to 2 parts water. If you have double cream, I would probably use 1 part cream to 3 or 4 parts water; that will give you a reasonable approximation of the consistency of whole or skim milk without making it too watery.
Please keep in mind that different recipes/preparations use milk for different reasons, so this isn't going to work everywhere. I certainly wouldn't do it in baking, but for many stovetop recipes, the main purpose of the milk is just moisture, so you're fine to substitute cream + water and it doesn't really matter if you're "exact" - which you can't be anyway.
Abstract: Ganache is delicious, but not everyone eats dairy. We examined whether coconut milk can be used for the creation of a non-dairy ganache. We ran a series of experiments. The answer is that, with some creative techniques, you can use it, but it does not come anywhere near to the real thing.
Introduction. Someone wrote a question on Seasoned advice about coconut based ganache and we got curious.
Background. We love ganache even more than pure chocolate, and make it often ourselves, so we think we have all the prerequisites needed to create and judge a coconut based ganache recipe. Our favorite ganache recipe 1 (which we used as a starting point) is: 100 g chocolate, 72% cocoa (only made from cocoa, sugar, and cocoa butter, no vegetable fat or emulsifiers); 100 g cream, at least 30% (non-UHT), 25 g butter.
Method. We prepared the base mix as follows: 25 g coconut milk were heated until boiling. They were removed from the heat. 25 g chocolate (72%) and 6g cocoa butter were added. Everything was left to dissolve. After 5 minutes, the mixture was whipped with a handheld mixer at the second-to-highest setting. This was Version 0. It was divided in two batches. Different versions were made from each batch by gradually adding guar (a knife-point-ful), egg yolk (1, in lieu of lecithine) and sugar (8g per batch). Each version was whipped for at least 3 minutes. The final versions (1c and 2b) where put for 30 min in the refrigerator, then taken out and whipped again. As there were no observable changes after the cold whipping, we do not treat them as a different version. We also made a small portion of dairy ganache for comparison purposes.
Results. These are listed in Table 1. The numbers in the ingredients columns of the table represent the order of adding the ingredients to the mixtures.
Table 1. Experiment results.
The final consistency of version 1c is documented in Figure 1. Note that the peaks do not drip and you can rotate the bowl at 180° without anything falling out. The piece of chocolate was added for color comparison.
Figure 2 shows a zoom of the blue bordered area in Figure 1.
Discussion. It is possible to make a profiterole filling with coconut milk based ganache, but it isn't quite like the real deal. Also, it resembles non-whipped ganache more closely than whipped ganache.
The basic ganache recipe can be done with coconut milk instead of cream and cocoa butter instead of butter. However, it tastes fattier than real ganache, and it is runnier. Unlike dairy ganache, whipping does not result in aeration.
We think that eliminating the cocoa butter altogether will somewhat alleviate the fatiness problem, but we did not test this proposal yet.
A thickening and or emulsifying agent can improve the ganache texture. Even though egg yolk improves aeration, is not a good choice, as it results in a slimy feel. Guar is a better solution.
The addition of sugar ruins the chocolateness of the taste. If bitterness is not desired, a sweeter chocolate (50%) should be used. Only dedicated sugar lovers or Americans should resort to adding sugar. In that case, it should be added before the guar (we would suggest dissolving it in the coconut milk before boiling it) as not to interfere with the texture of the final product.
Threats to validity. First, there was a single tester (the quasi-royal we). Second, this tester is already biased in favor of dairy products in general and specifically for dairy based ganache. Third, many interesting combinations (including xanthan gum and corn starch) could not be tested due to lack of resources.
Conclusion and outlook. We found the experiment very inspirating. It was also yummy. But we'd like to issue an warning: This experiment is dangerous. In the preparation phase, we discovered a penchant for guared coconut water deep fried in coconut fat we'd rather not have known about. Nevertheless, we are looking forward to offering the results for peer preview to a chocolate-loving lactose intolerant co worker.
References: 1 My own recipe book
Best Answer
Ciao Bella has an excellent 'coconut sorbet', that had the creamy quality of ice cream, without any actual dairy.
They released a cookbook a few years ago, and although I don't have it, I did browse through it in a store ... I don't remember the whole recipe, but I recall there being actual coconut meat in it, not just coconut milk. I don't remember there being any other starches or thickeners in it.