Yes, they do love water, and love to swim. It certainly helps that some of their natural prey (fish) are water-bound. They don't really "live" in water, though, as "semi-aquatic"
I was using semi-aquatic as they swim in the water put don't live in it but rather enjoy going in it would imply, and actually like a somewhat dry environment--along the same lines as a corn--because they're prone to scale rot.
For 2 garters, or 1 garter, or any number, I'd say as big as you can get. I have 2 juveniles (about the size of hatchling corns) right now in the largest breeder-style kritter keeper. That's about the same floor space as the 10 gallon. They do OK in this size, but the sooner I can upgrade them to my spare 20L (30x12"), the better. And this is just for two babies--like I said, they love their space! That and I, personally, like to see them with plenty of space so that they can still avoid each other if they ever need/want to.
Any of your biggest tanks (the 55 gallon
is going to someday(most likely)house 2 garter snakes, the 50 gallon, or the 40 gallon) should do just fine for two Garters. Just make sure they're the same sex and get along alright, just like anything else.