Hi,
I didn't fully understand 'planking on the inside of the hull' but probably it ment above the deck, don't know the hull, even though I have seen the ship just a few years ago.
Anyways, as sealing stuff has been mentioned, I would like to draw your attention to sikaflex products:
https://www.uksealants.co.uk/silicones-sealants/sika-sealants-adhesives.htmlWhich of them would be the best you may have to find out yourself, but they have silicon-less and paintable sealants. They are glueing strong but stay a bit flexible.
I think some of them can be used to glue in car windows.
Best,JoergP.S.: I hav no relation to Sikaflex, I just know that stuff from my 1:1 boating time.