I also have a sunbird, mine is the 201 Dual Console. Great boat for the price !! My boat came to me with 2 batteries which I do like a lot but it is an expensive add on compared to other options you have with what you are trying to accomplish. When I had my Bayliner 14' with 1 battery, I ALWAYS took the jump pack.
If you are not hard core fishing and running live wells, trolling motors etc etc then I suggest just getting a jump starter pack.
I picked mine up from harbor freight on sale and used a 20% coupon, cost about $30. Even though I have two batteries I still carry the pack onboard the boat at all times. I can not tell you how many boats on the water i've jump started with it and cars at the boat launch. It also has cigarette lighter receptable (use it for inflating the tube) and USB for charging your phone.
It looks like it isnt on sale at the moment, brand does not matter. This is cheaper than an additional battery, switch etc..
http://www.harborfreight.com/3-in-1-...ter-62306.html
I should add, my suggestion is only for jump starting your battery if it goes flat. With my West Marine 650 CCA starting batteries that are in my boat, I have never had to switch batteries or jump start. With fish finder, live well and radio on for a few hours. No issues. Most auto parts stores will happily test your batteries for you, leave nothing up for guess work when you are taking to the seas.
HAPPY BOATING!