It’s a neat idea, and it’s fresh. I saw a TV show last week (“World War II in Color”) that had an episode about British commando raids into Norway, Denmark, etc. just to keep Germany nervous. Apparently the Germans overreacted and stationed half a million troops in Norway all through the war, even though there were never any serious Allied invasions plans for Norway between winter 1941 and summer 1945. Some historians think that if Germany had had those troops available in France, they could have contained the D-Day invasion.
Part of why the British commandos could pull off their raids was because they had naval superiority. There weren’t any German commando raids into Britain that I know of. That’s why I’m a little uneasy about the “1 free sea move” thing. If I’ve got five destroyers in a sea zone and you’ve got nothing, can you still slip your commandos right past my boats?
Also, at 6 IPCs, I don’t think commandoes would see much use as marines or as elite infantry – you’d pretty much only buy them if you wanted to repeatedly conduct these fancy raids. Figure they get killed after an average of four raids – so you’re dealing (1d6 - 2) * 4 = 6.6 industrial damage for a cost of 6 IPCs. It’s not very attractive.
One tweak would be to have them deal (d6) industrial damage, but to give each enemy combat boat in the sea zone they move through a chance to detect and kill them (as well as the chance that local defenders detect and kill them).