Use marine rated treated lumber. It will last forever, it is easy to work with (square!!!), and will mate up with standard hardware. I probably spent $300 on my dock. It is small though...
Words of wisdom:
1. The MOST important thing: Get your posts in before the pond fills! Otherwise just make a floating dock. Floating docks have an advantage of always being level with fluctuating water levels some ponds experience in dry weather.
Now your posts do NOT have to be pounded into the basin of your pond. You can set them any number of creative ways. Some people have used 5-gallon buckets with a post set in concrete if the area where the post is to set is relatively level. I dug holes maybe 2 feet down, 2 feet in diameter, and then drove in 4' concrete spikes and set wire between them before filling the holes level with concrete. I then set and squared 4x4 post block foundations on top of the freshly poured concrete. Probably a full bag of concrete per footing. I then used double-dipped 'U' end brackets and screws anchored to the blocks and screwed into place. The dock is extremely solid, even with our icy winters.
2. If using wood for decking, use the best stain money can buy and stain everything before laying the decking down. Staining over your head while swimming and getting bitten by whirly-gigs is a lesson in what not to do. Otherwise use that fancy vinegar-treated decking that doesn't need staining. So far all composites I have seen get moldy and "funny", or get slippery as an eel over time. Also exposed wood can get splintery from constant wet/dry cycles without good treatment. I have taken to using indoor/outdoor carpeting over my cheap decking for swimmers. Good decking will make a big difference. Consider removable decking "panels" for easy maintenance.
If I were you, and you are physically able, consider doing the work yourself. Putting together a deck or a dock is pretty simple, and if you screw up, you can always tear down and fix it. You can put in a pretty darned good dock for $1K that will make mine look like a rotted log. Also 84 Lumber carries that marine rated lumber, and will deliver.
Someone posted a link to a site that sells dock and deck specific metal hardware. Pivot joints and the like. Consider a half-fixed, half floating dock to follow water levels if you think your pond level will fluctuate with dry summers. I wish I had, and I still might add a floater to mine.
