I've worked with similar (but smaller and more sophisticated) batteries to those used in the Volt; we used them for solar
The battery management system has, historically, been one of the big stumbling blocks. A fairly complex control system is necessary to keep big lithium batteries stable under aggressive charge/discharge cycles. These technical hurdles have now been mostly overcome.
The other hurdle, probably the more serious one, has been cost. There are few applications where $20,000 for a 5 kWh pack can be justified. When the carmakers figure out how to get costs down to reasonable levels- and they're getting close- we will certainly see more advanced batteries filtering down to us boaters.
However, I doubt that the batteries we will see in cars 15 years from now (and, therefore, in boats 20 years from now) will be based on lithium technology as we know it. While lithium is currently available in sufficient quantities to meet our present demands for it, there just aren't enough lithium mines and known reserves to handle a rapid surge of interest from the automotive community. Several carmakers are working on other battery technologies (such as lithium-manganese, already available for boats, or barium titanate ultracapacitors) that are cheaper and would be less vulnerable to commodity price
fluctuations if lithium comes into high demand.
Using the battery pack from a hybrid car as the heart of a boat's electrical system
is certainly feasible, but would require somewhat more engineering expertise than most boatowners, mechanics or electricians can bring to the issue at present. The possibility of burning out the pack (not hard to do with NiMH as found in most hybrids) or blowing up the pack (current lithium packs burn HOT and release extremely toxic fumes if they blow) is very real if you start tampering with these devices.
On the reliability
note: Lithium-polymer cells, as we used them in the solar
cars, do not last long (cycling right down to flat, then throw several hundred amps at it for ten seconds, then flat again, then a slow charge at 50 Celsius....). But the BMS can be programmed for whatever balance of performance and longevity you want. As for NiMH, well, last I checked Toyota still hasn't replaced enough packs from the first-generation Prius (1997-2001) to know just how long they'll go before they burn out. The Prius doesn't deep-discharge its pack; it cycles between ~40% and ~60% of peak capacity. I haven't seen anything yet regarding the charge/discharge profile the Volt will use.