Current is not an issue. A piece of equipment determines how much current it draws, not the battery.

Equipment designed to be connected to a boat battery needs to be able to handle the voltage applied to the battery during charging, either from the alternator or a charger. Most portable equipment isn't able to handle the higher voltages. You would also possibly be connecting and internal battery to the boat battery which could cause issues.

I'm not familiar with the Smartcast and how the boat-side unit is powered. Does it have an onboard battery?