What is the heaviest possible water using oxygen and hydrogen isotopes, and what might it's physical properties be? After a cursory internet search, it appears to me that Hydrogen-3 (a part of Tritiated Water) is the heaviest stable* hydrogen isotope. Oxygen-21 is the heaviest isotope of Oxygen with a half-life of seconds, and Oxygen-16, -17, and -18 are the heaviest stable isotopes (again, this is based on an internet search and may not be accurate).
So could (3H2)(21O) conceivably be a thing? If not, what is the heaviest water possible, and what would some of its theoretical physical properties be? Would the mass overcome surface tension? Would there be stronger or weaker hydrogen bonds? And so on.
The heaviest water obtained with stable isotopes would be $\ce{D_2^{ 18}O}$.  This compound is not well-charactetized itself, but Ref. [1] indicates that the melting point is only about 0.28°C above that of ordinary water.  The implication is that overall physical properties of $\ce{D_2^{ 18}O}$ are close to those of heavy water with oxygen-16($\ce{D_2^{ 16}O}$); apart from density the isotopic effect of $\ce{^{18}O}$ versus $\ce{^{16}O}$ is much less than that of deuterium versus protium/&quot;hydrogen&quot;.
References

F. Steckel and S. Shapiro, &quot;Physical Properties of Heavy Oxygen Water.  Part 1.  Density and Thermal Expansion&quot;, Trans. Faraday Soc. 1963, 59, 331-343.