The most obvious difference between the two is that the boa has a much shorter, stouter body, whereas a water snake is longer and thinner. The asper will also have a more triangular head shape than the water snake, in general.
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1.0 Solomen Island ground boa
1.1 green tree python
1.0 jungle carpet python
1.0 Irian Jaya carpet python
1.0 Boa constrictor imperator
0.1 Borneo short-tailed (blood) python
0.1 woma
1.1 Western hognose snakes
0.1 crested gecko
...and a recovering rescue ball python.