For the reasons that Lee mentioned, I'd suggest using a North American extension cord as it's much less cumbersome than having to use a Plug Adaptor in each outlet when using the French extension cord. When you're in Europe, you'll only need one Plug Adaptor, and on the ship you can connect it directly to the outlet.
Again as mentioned, the North American cord should be fine when connected to a 220 VAC electrical system, as they're normally designed with a safety margin and the insulation shouldn't be a problem (although these days with most products of that type manufactured off-shore and materials "cheapened-down", I sometimes wonder how much the quality has deteriorated).
If you decide to buy a North American extension cord (such as a power bar), it's important to note that it MUST NOT have any surge, RFI, noise filtering or other "electronic enhancements". It must be only a plain cord with a plug, copper wires and metal terminals in the outlets. THIS product is one that's designed to be used in both North America or Europe, although the cord is not very long. THIS is another product that should work, although I haven't tried it so I can't comment beyond the information they've posted online. One important point to mention with these products is that they have a three-pin (grounded) Plug, which means you would have to use grounded Plug Adaptors specific to each country you visit in Europe to connect them to the power point.
One important point to mention regarding extension cords is that the outlets are often close together. That makes it difficult to attach more than one larger power cube or camera charger at a time (ie: in a four-outlet extension cord, you may only be able to connect two power cubes at a time, leaving two outlets unusable.
If you have a lot of USB devices to charge, you could also consider taking a multi-USB Charger, as that will only require one outlet. There are lots of those listed on Amazon as well.
Hope this helps.