If your "house" battery is extremely low, and your "starting" battery is hot as a firecracker then when you go to turn the ignition on, the solenoid will energize and will throw 1000 amps into your dead battery. Deep cycle or not, this repetitive jolting of massive amounts of current is not hospitable to long life of the "house" battery. It will not last very long in this environment. It can also blow up the dead battery if it gets a shorted cell (nasty, very nasty). Also, an 8-volt charge on the house battery will pull your voltage at the starter down significantly. If you have a hard-cranking motor, this may be too much of a voltage drop to start your motor if it has vapor locked or is otherwise finicky. Without the switch, you have no way to disconnect the dead battery from the loop (besides taking the cable loose from it).
Secondly, in the case of a dead house and hot start battery, you will pass lots of amps through the cables and solenoid until the two batteries equalize out (which may take ten minutes or more). This may overheat the battery cables or the solenoid (fire and sparks and boats do not mix).
Your idea has merit, but I would prefer that you use one of the following methods:
1) CHEAPEST: Battery switch Off-A-Both-B. Swap to B when sitting. Swap to A to start if B is dead. After starting, swap back to B to charge with the 35-to-55 amp alternator/regulator.
2) Starter and motor wired to A. House and access wired to B. Battery isolator hooked to alternator to charge both banks when running. Solenoid that will strap A and B together when neccessary activated by a manual switch on the dash (all diesel boats have manually activated battery strapping solenoids).
various combinations of configurations can be used with Battery Switches, Battery Buddies, Isolators, Solenoids, etc.