Well there's nothing that's going to be "the same" as blood, but I've been involved with experiments where we used RPMI 1640 + 2% BSA to perfuse lungs/heart for several hours. The mice had heparin injected directly into the heart before the perfusion began to stop coagulation, and then the blood in the lungs/heart was flushed with the RPMI. I don't think anyone in our group ever tried, but I assume, that water would not perfuse for very long.

I would suggest a combination of the following: ~50% Heat inactivated serum (Atlas Biologicals, Fort Collins, CO), ~25% Opti-MEM + Glutamax (Invitrogen), ~25% animal buffer of choice (i.e., for amphibians use Holtfreter's solution, for avians use Tyrode's solution, and for mammals use Dulbecco's phosphate buffered saline). The percentages given above are only approximations. You will need to vary the percentages of the above solutions based on BOTH the pH AND the osmolarity of the specific animal's blood.

Tap water will lyse differentiated cells and progenitor cells, due to a lower osmolarity than the cells themselves, i.e., fluid will flow into the vascular endothelial cells and then into the surrounding cells to try and equilibrate the osmotic pressure with the cells, thus causing the cells to burst. The only cells that I know of that can withstand the effects of tap water and still survive are tissue-resident totipotent stem cells, tissue-resident pluripotent stem cells, and tissue-resident germ layer lineage stem cells.