Kotka is 1,5 hour drive from Helsinki (to east). Jamboree is a one day event.
Kymi-river is salmon and seatrout fishing mainly.
That time of the year is not really a salmon season. There are winter salmons still in the river, but no sense to fish them. Autumn is better for salmon fishing. (allso very crowded)
Spring is a seatrout time, but not so much fish in the river. Depends of the year. We've had better past thought...
First pools up from the sea are best for seatrout, but you must book your licence a week or two before. Maximum 8 rods per day. 24h licence is around 25 euros for first (private) pools. The upper river (public, Jamboree area) is around 8 euros and no need to book before. No rod limits.
I have to say, that many of my friends goes to Sweden (Mörrum, Dal) for spring fishing. There are lot of big salmons in the Mörrum river in spring time, while our rivers are mostly better after mid summer. Kotka's Kymi-river is a late season place for salmon.