In beetle caves scene 2, I can't swing across the river from the rock without..

Not all steps that I did may be necessary, but this is what I did...
After entering scene 2 go left past the cockroach hero to the boat. Allow the fish to flip you up to the rock and then defeat the mosquitos. A red cross icon with a white center has appeared at the entrance to the scene. Climb back up to return to scene 1. Once there go left and talk to the weak aminitan. Then go left some more to the valve. Hit it with Spore Grenade releasing a vent. Step into the vent to be sent to the top of the cave where you can save your progress. From across the chasm use Spore Grenades (ad nauseum) to defeat the beetle chief. He will leave an electrical device which you can now safely pick up by hopping over the chasm, walking left to pick up the chest, and then back. Proceed back down the cave to return to scene 2.
Save your progress again. Go right and down to the platforms over the green river, defeat the mosquitos again, and then hop to the second platform to the right. The Electrical device will be activated automatically to chase the fish away. Hop back toward the the scene 2 entrance and then to the left and down past the cockroach chieftain to access the boat (you could save progress again on the way). Now the boat will take you to the end of the green water and you can hop off to the right and continue on your merry mushroom way.

just upgrade agility as much as you can;
if that dosent work be a scout