When I was searching for a battery source a few months ago, someone in this forum turned me on to this company.
http://www.batterymart.com/p-12v-7ah...d-battery.html
The price was unbeatable and it was delivered in a few days. I use the 7ah battery to power my fish finder. There is plenty of juice to last a couple trips. It could also power some running lights if you plan to go out at night.
I am curious as to why you use a 12v battery for your bait tank. Most yakfishers who I know use a 6v battery for this. The pump runs slower but still fast enough and the battery lasts much longer.
If you are going to use both voltages, you should get a charger that can handle both voltages. Pep Boys and Harbor Freight tools are both good sources. Make sure that you get a charger that has indicator lights which show what is going on. I mistakenly bought one once that did not have any indicator lights leaving me to guess if the thing was charging and when the charging was complete.
Bob