Over the past few years there has been extensive commentary on the risks of skin fittings / thru-hulls / sea cocks / ball valves / pipe tails suffering from electrolytic corrosion - dezincification, that if left unattended can result in failure.
This well reported issue should hopefully have resulted in much better quality fittings being used, but it is wise to regularly check the condition of all fittings that are used that penetrate the hull, particularly below the static and active waterline.
This brief article shows some of the issues identified and solutions that owners have made on their own boats, but should not be considered as professional recommendations. If you have concerns about the potential watertight integrity of any fitting you should seek the professional opinion from a suitably qualified surveyor or boat yard.
More detail on the chemistry and choice of suitable materials can be found on internet searches for example:
Do not ignore your valves and fittings...
Often the weakest link will be the hose barb or tail, especially if it is thin walled, as it is likely to de-zincify the quickest, but often will remain visually hidden.
or where there are screw-threads.
Many skin fittings and thru-hulls though may remain in serviceable condition for many years. A qualified eye is required for a close inspection - after thorough cleaning of all surface corrosion, if there is no evident "pink" discolouration - which typifies the loss of zinc from the brass/bronze, it might be considered acceptable to leave in situ until the next inspection.
T ball valves have been known to fail and whilst this may well not result in any water ingress, the valve may not be turning internally.
If a skin fitting is difficult to remove from inside, it may be simpler and quicker to use a suitably sized hole saw to cut-off the exterior flanges. Hammer a wooden into the skin fitting from the outside, then carefully mark a centre for hole saw pilot drill. With care the hole saw can then be used to cut through the fitting flange. But stop cutting when most of the way through the fitting to avoid damaging the hull.
Ensure any replacement is either a high quality DZR bronze fitting such as these by Groco
The Tru Design heads waste ball valve above fitted with a 120 degree tail had another benefit - as it avoided the need to fit the original right angle bend that was prone blockages.
The following photos and notes are a summary of the reply by Caxton, the owner of a SO39i on the Jeanneau Owners Forum thread.
I replaced the raw water intake replaced with a Forespar Marelon Valve 931144 which has a 1" O.D. hose barb.
Old strainer removal
The hull hole needed slight enlargement
G10 epoxy laminate sheet was used as a backing plate plus lots of 5200 sealant.
Hose replaced with Trident Flex Wet Exhaust hose.
New Valve, hoses, and strainer 906062.
Credit to Jeanneau forum member "Caxton" for much of the above Forespar text and images