Oar extenders are a good solution if you don't want to cough up the cash for new oars, or aren't 100% sure if you want longer oars. They are awfully heavy though, adding 2' of aluminum so they are almost as heavy as an additional 2' of oar length. Once you get your oar length dialed, you'll be happier with new oars.
Counterbalances are very nice, especially with long oars. You position them (up and down the oar shaft) so that the blade drops right in the water. Keep moving them closer to the handle until the blade doesn't submerge when you let go, then I like to move them back a tiny bit for a bit more oomph in the drop. Yes, they add to the swing weight but overall make it much easier for longer oars, longer trips, or people that are just not quite as strong. Another drawback, your oars will sink like lead weights when they pop, so make sure you have tethers (so they don't disappear) and keep your oarlocks properly tightened/tuned.
The rafting counterweights have POS teflon set screws which when they shear suck to get all of it out of the threads (dental tool and patience). Do not let them shear! Use ductt ape instead once you find the right spot and just tape them to your oar. Be very careful with those. I carry a few spares just in case.
Also be careful not to get hit in the head with them if you drop an oar in a rapid. Wear a helmet, and have your passengers wear helmets. Watch the dogs too.
I do not use counterweights in big water (IV+) on my cat, just the raft which never sees more than IV.
I am a river, babe - I've got plenty of time, I don't know where I'm going, I'm just following the lines..... - "We are water" by Shaye