Buy your own and don't rent them. It is cheaper. Also note that some frequencies are illegal to use in some countries - and this can include both FRS and GMRS radios. BTW, GMRS radios also require an FCC license if you want to be legal for U.S. use.
Just please don't leave the radios on high volume with call alerts blaring. It is a cruise after all, and not a SWAT team convention.