Agreed - you could register the portable on a Ship's Radio Licence for the RIB, if you are only going to use it on the RIB, but if you want to use the portable on a range of craft, technically you should have a Ship Portable Radio Licence for it, which gives you a transportable callsign/T-number.
Given they don't cost anything, no harm in having both types of licence!
Note that these are not a "radio users licence", but the physical equipment licensing. The user licensing/certification is the RYA VHF SRC course/certificate, which is technically required (other than in emergency situations) before you use the VHF radio equipment.
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