Some Jeanneau's have suffered from water ingress problems at either the deck step and/or where the compression post is stepped onto the keel. This is normally due to water penetration that over time allows the wooden blocks that the factory used to rot and eventually collapse.
Indications that a boat may be suffering either of these problems, may include problems in keeping shroud rigging tension and possible leaks from forward windows in the area of the shrouds/masts.
The good news is that it is relatively straightforward to effect a permanent repair, and indeed many boats produced in the 1990 -2000 era's may have already have been repaired.
The problems are documented on the Sun Odyssey 43DS (early models) some of the Sun Odysseys such as the SO36.2 and SO45.2.
A good surveyor should be able to quickly determine if a boat has an existing problem, but irrespective if you consider your boat (or perhaps a boat that you are considering purchasing) has evidence of a problem, there are steps that you can take to reduce the risk.
In the photo below is a standard Sun Odyssey 43DS mast step with a number of separate electrical deck glands.
If any of the seals leak in this arrangement, then water will permeate into the wooden support that is moulded into the deck below the support.
There are a number of better solutions which will keep the number of deck penetrations to a minimum.
The images below are courtesy of "Freeflow" a Jeanneau 43DS owner on the forum, which show how he discarded all the poor individual glands and made a single conduit tube through the mast step base, similar to the routing on newer Selden masts - shown towards the end of this article.
An alternative is the classic swan neck design available from a number of suppliers.
and this patented Swedish Elvabro Cableport design.
More modern Jeanneau's may be factory fitted with a cable hose that is integral to the mast step, such as the Selden T Base design that provides a very large through deck conduit that extends high into the mast, requiring cables to be doubled back.
A similar arrangement, perhaps using a through hull fitting and tube might be retrofitted, the main issue will be gaining access inside the mast for the cable routing. The Selden mast has an aperture that allows for this.
If the mast step has rotted, the repair can be made from the outside as the detailed repair that "Geitz" the owner of a Sun Odyssey 36.2 has detailed here. Whilst potentially the repair might alternatively be made from inside the boat, this will require that the compression post as well as the mast to be removed. The benefits of the external approach is that both cutting out the old wood and then the fibreglass repair work should be simpler as gravity will assist not hinder, but the decision to opt for either an internal or external repair will vary from boat to boat, facilities and experience of the repairer.