Not sure of the layout of the boat, but if possible mount the battery as close to the original as possible. Make sure to use a good battery hold down and either use terminal covers or a battery box. They make a 2 battery switch that you can either run on batt 1, batt 2, or both that I would recommend. When parked with the boat off and stereo running, put it on batt 2, when going to start the boat if batt 2 is dead, switch to batt 1, start boat, and then put on both to charge them both. If the reason you are install another battery is because of the stereo, you should use a deep cycle for batt2. Deep cycle batteries are designed to be drained down and charged back up many times, while regular starting batteries are not. (One battery guy said 4-6 discharges and it is junk) Also make sure to use the proper size wire, too small may result in fire.
If you aren't in a huge hurry, go to a boat show and check out any boats that have a two battery setup with switch. You can get some really good ideas on where and how to do the job to look "factory".
