I have shared my entire life with Springer Spaniels. I absolutely adore the breed and will never be without (at least) one.
They are wonderful family pets; and active, intelligent companions.
Your questions:
"What's an average lab/springer spaniel puppy? Are they hyper? Timid?"
Generally, Springer Spaniel puppies are energetic and active. They need to be exercised quite a bit on a daily basis. They are also active indoors, as well. If you want a couch potato, do not get a Springer. If you run, hike, swim, etc.... Springers make excellent partners for those activities. They are known to be smart and loyal. They are eager to please, so training them often goes quite easily. But training definitely needs to be consistent and often, because they might get anxious if bored. I'd say they lean more to the hyper end of the spectrum than the timid end.
What's an average adult lab/springer like?
A Springer's average life span is somewhere between 12-15 years. They are prone to certain medical conditions, such as hip dysplasia, eye conditions, and ear infections. But a well cared for Springer can be happy and healthy well into his/her golden years. An adult Springer maintains a high activity level (I ran with my Daisy still when she was 12 years old... just not as fast or far). Once grown and trained (ie: once a puppy has fulfilled his/her potential), they are like part of the family. They love their people, and tend to want to be near their people as much as possible. I cannot say enough good things about this breed, but I, of course, am biased.
How much of a bond will a lab/springer make with one person? You might find the a Springer will pick a favorite person, although s/he will still love all members of the family. They bond strongly to their person. My beloved Springer, Jake (who passed almost a year ago), was my soul mate of a dog. He would not go anywhere unless I was going, too. My poor boyfriend couldn't get him to go out to potty unless I was going out, too. If my boyfriend walked him and our other dog, Jake would not go out of eye sight of the house unless I was with him. Our bond was very strong and Jake made it clear that I was his #1 person. He still loved my boyfriend, but he bonded with me immediately and was devoted to me. My parents' Springer, Winnie, is the same way toward my mother. And Waldo (who passed 3 months ago) also bonded more with me than my boyfriend. Our current Springer, Bubba, does not play favorites and is just a big love bug with everyone.
Do they like to swim? We live close to a dog beach.
I have never met a Springer that does not like water.
Good luck with your ultimate decision.
If you do seriously consider a Springer, may I suggest the Springer Spaniel Rescue Organization? I adopted my last 3 Springers from them. From the national link, below, you can find regional sites for your area.
http://www.springerrescue.org/
Also, here is a link with commonly asked questions and answers about Springers.
http://www.ariel-ess.com/essfaq/