IMHO the yeast is not dead but might be dormant. As has been asked in the comments above - there is no mention of priming sugar added to the brew before (or while) bottling. If the fermentation went to completion in the barrel then some priming sugar/malt will be needed to carbonate the beer in the bottle. If there was no sugar then there would be no secondary fermentation to speak of and the beer would seem "flat".
The rush to put the bottled beer in a cold cave (oh, that we all had such a thing...) straight after bottling may have been misguided. The bottles should optimally be held at (for example) room temperature (eg 20degrees) for some days - a week is not too long. Then the yeast can go to work on removing oxygen in solution and carbonating the beer. After that the beer can then be stored for conditioning in the bottles for (IMHO at least) one month.
However if the beer was correctly primed then simply removing the bottles from the cold cave, giving them a good shake and storing at room temperature for some time ( a week or two) should help correct the situation.
If the beer was not primed then you either have to re-open the bottles, add a heaped teaspoonful of sugar to each bottle and reseal it. Leave for the appropriate time again. Or live with flat beer and remember to improve the process with the next batch.
The dilution of the original brew is somewhat unfortunate but if only a small percentage (eg 5% or 1Litre/quart) of extra water was added then it should not be too bad (it is rarely "good"). If any greater amount of water was used to dilute then it may be better to use the made beer as a base for the next brew - Empty the bottles into the FV and add appropriate amounts of malt/haps/etc. there is no reason why that can't be done. And you wouldn't be the first to do it....