Now I have finally come so far that I can caulk the decks and the wheelhouse roof. I have chosen to use Simson MSR DC as sealant, a slightly expensive solution, but it should be the most durable. This is a modern MS Polymer sealant, in contrast to, for example, Sikaflex, which is a polyurethane.
I have pressed cotton in the joints of the old decks, the wheelhouse roof gets tape at the bottom of the joints. The joints has to be primed with Simson Primer P. I caulk between the planks first and then the edges in the second round.
And it's exactly the mess I was expecting it to be - it's a sticky black substance and it's all over the place in no time. But sometimes it has to be bad before it gets good! I use an air seal gun - otherwise, I'm sure I would end up with severe tendonitis - but it requires a little practice before the dosing becomes appropriate. As a result of that, I have used too much sealant for the foredeck, so I must have ordered something more.