weight drag

Question
On the erg a heavier weight is a benefit, but why isn't it on the water? Of course I don't mean excessive fat as that is a dead weight which would slow the boat down. But people say that being heavier is a disadvantage because of the drag this causes on the boat. However, aren't there different types of boat depending on the persons weight? So if a 75kg person was in a boat for people of 70-80kg and a 60kg person was in a boat for 65-75kg (if that even exists) then wouldn't they have the same drag from the displacement of water and thus not have a difference (not taking into account different skill levels, size; air resistance, power)?

This might just be a silly question, as I am just confused :L

Answer
You are correct that the reason that why heavier rowers are relatively slower on the water is because they displace more water and therefore have more drag to fight through.  You are also correct that there are boats that are made specifically for heavyweight rowers to make it easier for them to row.  These boats, however, are not made to sit the same amount of distance UNDER the surface of the water.  They are made to sit the same distance ABOVE the surface of the water.  For example, a lightweight boat may sit with 12 inches of its shell underwater and 12 inches of its shell above water.  A heavyweight boat would be made to still sit 12 inches above the water, but in order to do that, it might need 15+ inches of its shell underwater.  Basically, the boat designers are adding in extra area into the boat to achieve the buoyancy required to hold the heavier rowers the same amount of distance above the water so that they have room to get their blades in and out of the water.  On top of that, heavyweight boats are usually a little wider than lightweight ones to account for the rowers' larger size and to help them more easily set the boat up.  Although, a wider boat achieves these things, it also forces the rowers to have to displace more water on a horizontal plane with each stroke and therefore slows the boat down just a bit.