The space where a generator would normally go is the right place to store your batteries as the weight and balance considerations have already been made by the yacht manufacturer's naval architects. 

The forward locker is a very common location for the Genset,  there is no problem locating the batteries there provided they are well secured to prevent movement.  You can usually seal the top of these slightly better.  That is something you can request from your installer.

Marine Batteries like these are made for a moist environment. Certainly they should be covered from splashes but after that it is really your choice of installation location.