Hi Hamlet,Yes, pskmail can be used to access the internet. I use it on my boat and many others use it on theirs, its actually a very nice fit for the requirements on a boat. Regarding hardware you do need a transceiver, a soundcard interface, an antenna and perhaps a tuner. I use a netbook on board as it uses 12 volts and very little current. There are multiple choices for the rig, I suggest you look at ebay for prices of used rigs. I see an Icom 718 there now at $599 and an ICOM 706 at $400. A Yaesu 857 is a nice rig and its available at around $800. Of course you can spend thousands of euros and get a very, very nice rig but you really do not have to.
A steel boat sounds great, many have plastic boats and the big issue then is to create a working "earth" counterpoise for the antenna. Using a good tuner its possible to tune almost anything but the radiation pattern is another thing entirely. With your steel boat you should get a very nice signal using the wire you mentioned. I would put a remote controlled ATU on deck (Icom AH-4 or SGC-230) and connect its earth to the boat and use that wire as antenna.
Yes, you do need that ham license. I don't know how hard that is to get in France but it should not be very difficult, the things you learn for the exam are things you will need to know anyway! I got my license 25 years ago and I'm not french so I'm not your best source there. But, google should help you find something useful. I studied on my own using a book I got from the local library, there are also courses you can attend.
If you need further information then we are here to help! 73 de Per, sm0rwo