FreeCad and Inkscape are not realy a good team

Maybe there is software well suited for custom SVG suited for SH3D cutouts, but I have not found it. In your case, where almost all the co-ordinates are easy to measure (except maybe parts of the C, Bézier curves) my guess is still that it will be both easier and more accurate to work with the SVG path directly. Hence the cheat file for visualisation above. If opened in Inkscape, you can also import a background image to help.

25m long and 5m wide

Is pretty big... You probably know this, the ICC pleasure craft certificate is not rally standardised, so you may run across problems on a European tour. I do not know various European licences, but here anything above 50ft / 15 metres will requires a much more demanding certificate.

Btw, in addition to your boat line-up in thread,8749 - there is another option if you reallly want to design your own boats. Several manufacturers will sell pontoons on a frame, on which you can construct whatever you like, thread,6956 (and a picture of a live example in thread,6916). While the pontoon construction may not be for offshore use, they are surprisingly seaworthy and quite stable because of their catamaran/trimaran design. And, of course, they do not have to look like boxes...

ok