Michelle,
We have 4 labs they all come from heavy field/hunt test lines. Picking a breeder is the most important thing aside from educating yourself about the breed. Just because a breeder come highly recommended see a few. There are a lot of great breeders in the NW.
We have got 2 of our dogs out of state for the different bloodlines.
Hunting lines are important however, for a first time lab owner I would be careful in what you purchase. You say you want a mellow dog, field labs are bred for drive, high energy and desire. There are some field labs that have traits very desirable to the field trial guy but, not the best fit for a first timer.
Hunt test dogs can be just as high energy but just the preference on what the owner want to compete in.
Of course any dog can be trained to be obedient. It just takes reps. and being consistent.
It's not the age of the dog as far as it learning to swim as it is the weather conditions. We got a female in April and she was swimming right away. When they are little they cannot manage their temps.
Get the Wolters book, another one we have is the tritronics total retriever, I like that one better but, that's just me. Wolters sets up time lines that you have to ignore. Every dog is different and so, it's a good book but, you have to ignore the time lines.
Do you plan on keeping it in the house?
As far as the CH dogs go, I will say they "can" have the attitude that they just want to lay around. They don't all have the breeding that includes the high drive. It would be harder for the average person to train, if you got one with out the desire/drive.
Just remember you can't put it in them but, you can take some out.
Feel free if you have any

to ask me. If you want breeder suggestions, I will give you some however, I won't air them publicly. There are some too I can tell you to stay away from!!
Good luck in your search. If I see something that I think is fitting, I will PM ya:)