Boating is very popular in Finland but RIBs are rarely used only for cruising. They are mainly used by army, frontier guard, police and SAR duties. Some dive clubs use also RIBs.
You asked locations, I guess you ment locations for boating (RIBing). From my point of view best areas are south and south west coas (espesially Turku archipelago), lake Päijänne and lake Saimaa.
Crossings from Helsinki to Tallinn (Estonia), Hanko to Tallinn and Turku-Åland-Stockholm (Sweden) are also popular.
For example Helsinki-Tallinn is 45 NM and RIB from 4m size should be enough.
I'll be pleased to tell more If you need any info.
There is the "Boat 2002" boat show in Helsinki next month.
I'll find out if there is any company that for example can rent RIB-boat.