The mangrove snakes are certainly inclined to bite and while they incontrovertably have a venom, the delivery and secretion architecture aren't nearly as advanced as say another Colubroidea such as any of the various Elapidea genera. So, the maximum a bite by that Boiga species with good and prolonged chewing would produce would be mild systematic neurotoxicity usually manifested as slight giddiness and an absolute ripper of a headache. Some of the other Boiga species may be more or less warm.
However there are a lot of unknowns out in the ole Colubroidea. Many totally unrelated lineages (as different from each other as an elapid is from a viper) have been dumped together as 'Colubrids'. This group doesn't exist as a natural group, rather being very very diverse. Some of the other lineages contain members that can be much more toxic. But most of the major Colubroidea lineages have at least one dangerous species in it.
All the best
B
Venom & Toxin Database