The FCC rules state that a "volunteer" vessel (i.e. recreational) does not need a FCC station license and MMSI, if not going to foreign ports.
It says explicitly that international waters, and Hawaii would count as domestic, i.e. the non-official Boat-US MMSI is fine.

What is the general take on this? - An un-offical MMSI does not show up on the international registries, changing it for existing equipment is a hassle so...