Don't have experience of boating in Rauma, but done a bit of it on the south coast.
Generally speaking fairways are well marked and maintained. Outside fairways You kind of move on Your own risk. Most of the myriads of rocks and stones are marked on sea charts/plotter charts, but not all, and on shallow areas irrelevant for commercial shipping data might be less accurate. So better take it carefully when exploring waters outside fairways.
Baltic sea is not at its best these days, the water is not as clear as it used to be, so that does not help much. No tide, but wind/weather do affect the water level. On the good side, daylight period is very long at summertime, hardly ever need running lights. Another good thing is that except for restricted military and nature protection zones, you are pretty free to visit any island or beach, never mind the ownership of that land. Common sense is required, but in principle its ok to berth the boat on a private own island as long as not disturbingly close to a building.
You don't need to have any certificate to drive a normal leisure boat. But any boat(local) with a LOA more than 5.5 meter and or HP above 20,4 need to be registered. Insurance is not compulsory. So a SIB with a 20 hp does not need any documentation. Rauma is a tiny place so don't know if they have boat rental there. But i am sure some of Your local colleagues owns a boat and will take You out if they know you want too.
fun on a boat is inversely proportional to size...sort of anyway