What does this Swedish sonar image depict?

I've based this on the linked Swedish National Heritage Board (RAA) information and my experience in the marine geophysics sector:

• This 'object' has been imaged in at least one survey, performed in 2007. No other datasets that cover this area are noted.
• The National Heritage Board is the authority in this matter, and unless good contrary evidence is provided or another survey performed, their interpretation of the site should stand.
• The National Heritage Board has classified it as "Skepplanda 10002" with related notes (Google Translate):

Fartygs-/båtlämning
Detta är troligen en fartygslämning som misstänks vara gammalt.
Enligt Sjöfartsverket så är detta troligen ett vrak som kan ha en viss ålder.

Vessel/boat remains
This is probably a shipwreck that is suspected to be old.
According to the Swedish Maritime Administration, this is probably a wreck that may have a certain age.

• The data is likely to be from a MBES due to the way the beams line up and the data is presented (which is actually very poorly).⁽¹⁾
• With a typical colourscale, this would be shown blue to red with blue the deepest and red the shallowest. I've attached a colourbar to illustrate this range. In the OP's image, red would be 9.2m depth, and green 9.7m by my estimate from the measured height and the sample classification.

• The MBES is most suitable for determining the height (depth) of the water column. While its positioning is good, its resolution normally isn't (compared to some other sensors at least). I wouldn't do archaeological interpretation based on MBES data alone, but, as I've specified, the RAA are the local experts to whose judgement I would leave this question (local expertise, prior experience, etc).
• A MBES has fairly high accuracy with a typical depth error budget of generally less than 1% and minimal positioning uncertainties. However, data from 2007 would have been worse as also shown by the fairly few sample points that cover the object itself in the image.
• This object was measured (using the OP's imagery) to have the following properties:

Width (m) 5.5
Length (m) 15.6
Direction 36
Dive depth 9.7-9.2
Depth 9.7
Height above the bottom 0.5

• In other words, the confirmed dimensions that apply to this object are 15.6 by 5.5 by 0.5 metres (length by width by height).
• The image looks as if the outer hull is intact. I would venture a guess that the vessel sank to the bottom of the riverbed, perhaps with some damage to the upper structure (impossible to determine without more information). After that, the river has slowly been moving sediment into the wreck so that the central parts of it are now filled in with the clay that the OP describes as the river's environment.
• The outer boundaries are fairly sharp (high gradient) compared to the rest of the structure. It could be something more like a barge which never had much height change at all across its structure.
• Wild Speculation: Without knowing more about sediment transport in that river, it's difficult to say much, but normally rivers carry quite a bit of suspended sediment. The 36° alignment is kind-of in line with the river as well, so it may have been scouring the wreck deeper while infilling it from above with sediment.
• This is where more data that would characterise the actual sediment grains and whatever the protruding objects are would be very useful.
• For these dimensions and an anthropogenic origin, a "boat remain" interpretation makes sense as there are not many other things of this size. Yet, and I should stress this again, I would be very reluctant to do this myself without other datasets.
• With regards to this possibly being a cannon or gun, (beyond the question of dimensions) if it was a gun, it is highly unlikely anything would have happened to it while in water—i.e., unless it exploded and sank because of that (in which case I would expect other records describing this event) or unless someone purposefully went to break it apart after its sinking, the gun would have stayed in one piece. Also, bronze should remain intact in fresh water and not corrode which would be the one mechanism in saline water by which its dimensions could shrink.

_{(1) As an aside, normally this type of interpretation (archaeological) wouldn't be done from MBES data alone. Most archaeological surveys involve more sensors: at the minimum, a side scan sonar, which would provide much more information on what's actually on the riverbed and not just the height/dimensions of those objects, and a magnetometer, to measure a potential anomaly's effect on the local magnetic field.}