I don't think you should have to be on plane to get the water to drain, it should drain at trolling speeds.

My dads 2004 21' Campion will sometimes take in a little water if three people stand at the transom and the boat is not moving. It will drain out as soon as it starts moving or people move forward.

It sounds like you may have a good fight ahead with the dealer and manufacturer.

Post a this qusetion on iboats.com and/or TheHullTruth.com I'm sure you will get a lot more replies.
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Damon