Hi Steve,
Regarding a 'new' charger; if you're going to run the boat on 20 cells wired in series, it is good practice to charge the cells in the same configuration as you use them, aka in series.
This avoids nasty rapid equalisation rushes between the packs when you connect them. Make no mistake, even with NiMH packs you can arch-weld...
This requires another charger as the Imax B6, as this charger only handles up to 15 cells NiMH.
If you're not planning on changing to Lipo's (which you no doubt will after the first runs on NiMH...), I'd search eBay for a charger like the Graupner Ultra Duo Plus 30, which can handle up to 30 NiMH cells with a charge current up to 7A.
This and similar chargers are being sold relative cheap, as they can't charge Lipo's as well as the new generation chargers.
There are a few on the German eBay: at the moment
http://www.ebay.de/itm/Ladegerat-Graupner-Ultra-Duo-Plus-30-/170984945545?pt=RC_Modellbau&hash=item27cf7f3789You'll need a mains supply of 12V 15A minimum.
As I've mentioned before, please change the Tamiya connectors for something able to handle the current, like Ø4 mm gold plated connectors.
While the Tamiya do smell nice when the plastic melts when the connectors overheat, the possible shorting and burning following immediately is less pleaseant...
The picture doesn't show how high the swimming platform is above the bottom; if it sits too deep, it'll act as a (huge) trimtab, rendering your fancy controlable ones inert.
With two motors you'll not experience torque roll, so operating them simultaneously isn't an issue.
Testing will show the waterinlets are set rather (too) deep and will add considerable drag, specially if you pick up debris.
The intake may also be a little overdone.
Regards, Jan.