Plymouth Park is on the Washington side of the Columbia River just the other side from Umatilla Or. Take highway 14 West just before the bridge over the Columbia into Oregon. About 1 mile down take a left. Down thru what is refered to by the natives as Plymouth. As you round the bend you will see the park entrance on your left. Don't take the left at the turn as that is a dead end, and a bear to turn around on. Watch for the park sign.
As far as camp sites the best are on the south side as they have the most shade trees. By the same token those are the trees that fall in a good wind storm. #10 thru 24 have sewer hookups, with some exceptions, when kids throw rock down the pipes. 14,15,16,17,18,19 are probably the most popular. But if your only going to be there a couple of days and don't mind stopping to dump going out, 1-8 are all very nice. What the heck, I like the whole park.
Sprinklers come on Mon thru Thur night so stay off grass with tents etc. or get wet if you don't mind.
As far as I know, dogs can swim anywhere they want, it's a big river, but fairly swift in spots. Nice swimming and picnic area in the day camp for kids, when it warms up.
Lots of good fishing and boating.