Yes, a voltmeter could be used as the required hydrometer. There is nothing in the event description that would preclude such an approach (use of electronic instrumentation). The competitor should be prepared to explain where the various parts come from and demonstrate that the student's device is not from a commercial device for measuring water quality.
There are commercial devices that can measure several parameters (pH, temperature, conductivity - which can be interpolated as far as salinity) that could be called a "multimeter" and would not be permissible. If "multimeter" means something else, like a generic electronic device, then it may be incorporated into the student built device. Students should be prepared to explain their devices.