I am very surprised 2A was not enough.
If it needs 12V DC wire up suitable plug to a cig lighter. The boat battery will provide all the current the player needs.
The voltage will be slightly higher than required when the boat battery is charging,. This is unlikely to cause any problems, but there is a small risk it will damage the unit. If you use an unregulated 12v wall adapter you are taking the same, or greater risk anyway.