If 'Water is H2O' is metaphysically necessary but not knowable a priori, then the theory containing 'Water exists' modally entails that hydrogen exists.

If the notion of ontological commitment is to play a role in theory choice or clarifying ontological disputes, then the ontological commitments of a theory must be rationally accessible; failure to acknowledge the ontological commitments of one's theory must be a rational failing (see Michael 2008). That puts the following epistemic constraint on the entailment relation that is invoked by an entailment account of ontological commitment: the entailments of a theory must be knowable by any ideally