Hydrolyzes 3-hydroxyisobutyryl-CoA (HIBYL-CoA), a saline catabolite. 3-hydroxy-2-methylpropanoyl-CoA + H2O = 3-hydroxy-2-methylpropanoate + CoA + H(+) Amino-acid degradation; L-valine degradation. Belongs to the enoyl-CoA hydratase/isomerase family.