I presume that the boat concerned is a GRP one.

Irrespective of the original method of securing them,  I would be inclined to try taking advantage of modern materials,  in the form of flexible sealant-adhesives.   Silicone sealant is fairly ubiquitous,  and I am sure would do the job;   I have known other owners strongly recommend polysulphide sealant-adhesives;   and I myself currently tend to favour CT1,  a hybrid polymer sealant-adhesive https://www.ct1.com/our-products/ct1/.   An internet search on the name CT1 will reveal plenty of retail stockists,   or you could find their local distributor through their website.

Another very well-known brand is Sikaflex,   and if I remember correctly they offer a variety of products,  to suit different applications;   so have a look at their website to choose the most appropriate product  https://www.sika.com/en/brands/sikaflex.html.

As I see it,  use of an adhesive of any of these types will enable you to bond the timber in place without needing to pierce the top of the buoyancy tank with screws,   and hopefully the bond will be sufficiently secure for all normal use while at the same time being weak enough to allow you to remove the timber again in the future if you ever need to do so.

Hope this helps,

Oliver

Sean,

To give you an answer I’d need to see the situation. Photos please.

thanks,

Steve.