This trip is highly recommended, and the perfect time to go is when it's starting to warm up and there's still plenty of water in the creek. There are several opportunities to splash around and stay wet. The paddle to and from the creek (4 miles each direction) is pretty reasonable, too. I'd imagine that timing it to where one of the paddles is either during dawn or dusk would make this trip extra memorable.
Brian nailed the description, but I wanted to throw in a few points of my own.
From the cove where you "park" your boat, there is a cairned route that leads all the way to the cave. We followed it on and off, but I preferred the creek almost the entire time. The route is very overgrown and, this time of year, the catsclaw is in full force along with the snakes. In fact, as we began our hike up the creek, I said to Brian, "dude, I don't see how we could NOT see a snake today." I had barely finished my sentence when he exclaimed "snake!!!!" It was a little guy- not a rattler- and darted into the bushes as soon as Brian saw it.
The "narrows" section comes pretty quickly after you begin sloshing up the creek. Here, pools begin to form and finding a bypass is necessary if you want to avoid thigh-deep water in spots. Heading upstream the creek just keeps getting better and better. I was constantly impressed with the scenery around every bend. This creek is just a trickle during the warmer parts of the year, so the riparian vegetation isn't too thick, allowing for constant views of huge sloping canyon walls dotted with saguaros. Throw a slick granite waterfall in the foreground and you've got prime desert scenery.
After about a half mile we came up on a man and his two children playing in the creek. I thought "hmm... that looks fun, but why this spot over the several great swimming holes we already passed?" That question was soon answered when we rounded that last turn to an awesome 20 foot waterfall spilling over a granite overhang, complete with a large alcove behind it. This would be a great place for a shower on a backpacking trip. We stood and talked to the man for a while, sharing kayaking and hiking adventures, then continued on our way.
A couple more impressive pools and cascading waterfalls are passed and then soon the vegetation thickens... It seems obvious the creek is perennial in this area and travel becomes a bit more difficult. It reminded me slightly of Coon Creek in the Salt River Canyon Wilderness. Up the cascade, under the fallen tree, over the slick boulder... Anything to avoid the thick riparian vegetation on the banks. After about .8 mile, soon after a narrow canyon enters from the right, you can see the west (left) canyon wall of Alder Creek quickly fall back. Start looking on the right bank for some barbed wire strung between a couple trees. This is pretty much your exit point. If you're lucky you can go a few more yards and spot a faint path heading out of the creek. Either way, head up and look for a giant overhang on the east wall of Alder Creek. Bushwhack that direction and you'll probably pick up a footpath that will get you the rest of the way there. In the cave you can sit and have lunch and ponder the various rusted trinkets laying around and especially that odd painted looking rock face directly above you. What the heck is that?
During our trip, after we hiked back to the kayaks I went for a refreshing swim in the cove. Then, on the paddle back to the truck I spotted a rope hanging from a large cottonwood in a cove on the south side of the lake. Being a sucker for rope swings, I had to try it out. Definitely one of the better rope swings I've tried.