ohms, parallel, series, 6 woofers

unfortunately you won't be able to use one amp for all the subs.  There are two issues you'll run into.  In your application, you'll need every last ounce of bass from your 2-12's, and you'll need to match the output of the 8's and the tweets in the tower. 

One other problem you'll run into is that you have to wire up all your 12's and 8's to make sure you have a balanced load.  If your final impedance of the 8's are different from the 12's, then the power will not be balanced between the two systems.

I've worked on tons of boats, and I've tried what you are doing in a real world application.  I had a customer with a single 10' and twin tower speakers that were bi-ampable.  The same scenario you are doing, but a smaller system.  He had a few amps he wanted to re-use and after insisting that he purchase new amps or rethink his idea, I ended up installing the amps just like you are attempting.  The result:  HORRIBLE sound.  I spent a few hours trying to tune the amps to make it work and I could only get one of two results.  Either the bass was non-existant, but the towers sounded decent, or the bass was strong but the towers sounded like "pip fartin' on a snare drum"

You're best bet is to use a seperate amp for the towers and one for the subs.