The following answer is based on your being physically present; but I see your wife and child will e resident for 183 days, making this your domicile. Gotcha.
1. Currently UK nationals moving to France are liable for wealth tax (impot sur la fortune) on the net value of their worldwide assets, including the value of any real estate or rights in real estate outside France.
2. If you work for a UK shipping company, then the current tax treaty allows you to escape taxes in both France and the UK, as you have been doing.
This will all change in mid 2009, when the new UK-French Tax treaty takes affect. BUT, you will be gone by then.
In the mean time, if you have purchased a home in France or established a physical residence there for your family, for tax purposes, France will consider this your domicile.
But you are working off shore, and if the shipping company you are working for is not a UK employer, then you would have to pay taxes to France.
If you are a seafarer IN TERRITORIAL French waters you would pay income tax to France.
If you are a seafare NOT in territorial French waters, you would not pay income tax to France because of the French exclusion of seafarer income earned while outside French territorial waters.