A few things come to mind on this type of issue and I have had the in my 25 as well .
First, I doubt there is a seal or supposed to be sealed. There should be some type of ventliation flowing through the nooks and crannies to keep mold/mildew down.
Second, you might have some mold/mildew and to help resolve that, it is a lot of work to remedy and have to try different things.
In my case first I cleaned the bilge out with some heavy degreaser, but if in the water, that might be problematic to get it all cleaned as well as can be done on the hard with drain plug removed. After I flushed the bilge areas, I used my air compressor spray gun on high pressure, over 100psi with some Enzymatic type solution and just sprayed behind the panels, in the bilge so as to fog the areas I wanted to treat.
Once I let that dry, with aid of box fans, I then used the Starbrite NosGuard treatment.
Then, it seemed to take care of the issue.
If have black mold, you need to work on that. In my case, some areas were pretty bad so I used some special products to kill the mold, then used a bleach solution to remove the stain, then rinsed and again, used box fans to dry out for a couple of days .Then I can tell in the future if the mold is trying to return.
If so, I treat it again and repeat above.
I finally have control again and the odors are gone.
Also try to remove any leaks out of the equation, especially from the head area. .
