LFT
The Q400 does have a ditching dam system, although on this version the dams are fitted to the forward pax door and the forward right emergency exit. As the 400 has a rear pax door and a rear service door, there is no need for additional emergency exits located near the wing (at least according to the certifying authorities.
The theory behind this arrangement is that in a water landing, the two rear exits are expected to be unusable, but with the DD in place, the two forward exits should be serviceable. I don't think this has anything to do with a higher gross weight, but more the length of the airframe and the 'normal' weight distribution.