I also say you could have some wet foam. Look under the floor boards. This is common and I'm near certain it's your problem. Hewescraft craft installs the foam so the boat will still partial float for a few days after getting a massive hole in it. I read that it saved life's and they won't make with out it. The same foam is under the gunnel. Fresh water weights 1kg/L and it dosent take much to add a lot of weight.
You can read lots on this topic at Hewescraftowners.com it's a great forum with lots of info from owners.
I have a 190 searunner ET with a yammy 150, kicker, 2 battery's and it never sits like yours, even with a cooler packed with beers on the transom. Only time is when I have 100's of pounds of bottom fish after a great buddy trip. Not sure if the ET makes that big a difference with rear floatation?