Not all Battery Isolators are Created Equal
To expound on the answer from Move More Comments Link To Top, there are some differences in charge isolators that should be made clear. Some operate with diodes to split the charge current to each battery, while not allowing back-feed from one battery to the other.
In principle, this works well enough, but in practice, there is a ≈0.7V drop across all diodes which prevents either battery from seeing a charging current strong enough to de-sulfate the lead plates in the battery. This will lead to early battery failure and is the main reason many people believe that 'boats eat batteries."
If, instead, one were to have a relay that would selectively connect the batteries during charging and disconnect them during use, that would present the ideal solution. As it happens, some other engineer thought the same thing and developed a product around that concept!
The one I have personal experience and recommend with is from Blue Water Systems and can be found here. The principle of operation is pretty simple: if the device detects voltage above a threshold, it connects the batteries with a direct connection via a solenoid. If the voltage drops below a different threshold, the device disconnects the batteries.
There are a couple options available as to whether you might want to provide charge system current to the aux battery for extended use, or would prefer to isolate the starter battery during starting to protect delicate devices on the 'house' system.