Boil

A designated area of liquid up to a max volume of twenty gallons instantly turns to its gaseous form upon the caster touching some of the liquid. A small amount of liquid removed from a source can be touched and still work to turn up to twenty gallons of the liquid from the source into its gaseous form despite them not being connected physically. 

The caster makes a spell attack roll on any individuals exposed to the liquid the caster touches and boils. Anything in the liquid that has vulnerable areas such as skin or part of their base form exposed to the liquid takes 3d6 radiant damage upon a hit (even on a hit, if the target is magically or physically shielded in such a way that the liquid doesn't touch any part of their form then the spell has no effect on them). 

The designated liquid must be freely exposed to the air and not contained. For example, a waterskin with the end tied off wouldn't be affected, but an untied waterskin with the liquid exposed to the air would be. If the caster were to spray some of the waterskin's contents onto several individuals and then cast the spell and touch some of the waterskin's remaining contents then this would serve to allow the caster to make spell attack rolls against any individual where the liquid touches them in a vulnerable area (whether the liquid hits them in the first place would be another attack roll and would require the caster to take an action to do so etc).

A person with a bleeding wound would count the same way as an untied waterskin but automatically would take the damage (barring some form of magical shielding preventing magical attacks) must also make a constitution saving throw (since blood is a liquid). On sucess the individual loses no blood and takes no extra damage or effects. On failure they would lose blood up to the discretion of the DM (size of the wound, amount of blood flowing, etc) and the resulting effects of blood loss would be at the discretion of the DM. Any creature with fluid that leaks from its base form operates on the same principle whether from a wound or not. A target at two thirds health or higher makes the constitution saving throw at advantage but any target at one third or less of their hp must make the roll at disadvantage (they must have leaking liquid from their base form for it to work). A target above one third health but below two thirds health makes the constitution saving throw normally. The caster would also need to touch the leaking liquid from the target in some manner (perhaps by collecting a drop or two of the leaking liquid from the ground for instance).

The above mentioned examples illustrate rules to guide the caster and indicate that you can get creative with the spell but are by no means the only way to use the spell either for utility, or for offense, or for defense.