Early versions Yanmar 4JH fitted to quite a lot Jeanneau sailboats including the SO45.2 and SO49DS had the raw water pump fitted facing sternwards below the alternator.
This made replacing the impeller quite a challenge to do even in port.
Newer models of the 4JH have the raw water pump relocated to the front where servicing is much simpler.
The resulting problem for impeller replacement on rear facing pumps is discussed on this forum thread, from which this Hints and Tips article has been based.
In summary if you have one of earlier engines there are some steps that may make the process simpler.
1) It is wise to switch off all batteries.
2) Some people have found removing the alternator is helpful and some mechanics even recommend pulling out the entire water pump body, before impeller removal.
3) Use an impeller which has an internal thread, this will allow the simple bolt puller to be used to extract. These can be either bought from Yanmar or can even by self made using this guide from Mistroma. DIY extractor
4) If the impeller is already fitted and does not have the internal threads it may be helpful to use either a special impeller puller or possibly a pair of right-angle pointed nose pliers
An alternative option is to permanently replace the waterpump with a crankshaft driven unit on the front of the engine. This is a project for reasonably competent owners or engineers, but the steps are fully described in this pdf download.
Link to Water pump replacement article by Arthur Boas