The following images and notes are based on the detailed repair that Magnus Lundberg the owner of a Sun Odyssey 39i originally detailed here on the Jeanneau Owners FaceBook Group.
The 42i and 42DS have a similar construction.
The teak on the lowest step had come loose in one corner.
At the bathing ladder there is only about 1cm of plastic until you get to the teak and therefore it started leaking.
It is not plastic deck under the teak so any leaks will drip into the boat.
I used a sharp knife and some things to lift the teak more and more while cutting the sika underneath.
Took me about 5 hours.
The plywood was rotten in that corner so I have cut out a large piece to replace.
Lots and lots of Sika 291i...
Something to spread the Sika...
The first pieces added... these are the ones that is essential to remove if you have leak.
The 2nd step can be used to press against with clamps...
SO39i - Magnus Lundberg