UHF = ultra-high frequency = frequency range from 300 MHz to 3 GHz
VHF = very-high frequency = frequency range from 30 MHz to 300 MHz
Marine VHF radios in the Americas operate on specific channels between 156 MHz and 162.5 MHz. There are no primary allocations in the United States for marine radios at UHF, although UHF portables are often used dockside. The main frequency range typically referred to as UHF in the Americas is 450-470 MHz, but those frequencies are allocated to business users, public safety agencies, etc. Attempting to use in the Americas a UHF radio from another part of the world would be a bad idea, as it could easily end up interfering with local police, fire, ambulance, etc. communications, which would be likely to bring swift enforcement action from the U.S. federal government.
So, no, a UHF radio is not the same as a VHF radio, and no, a UHF radio would not be suitable for use in the Americas on a boat. VHF marine radios are mass produced and preprogrammed to the standard channel assignments, so they may be obtained quite inexpensively.