You get FICA exemptions when you sail foreign as an American. But, that can hurt you in the long run, if you don’t make it up.
Agree, bad idea as you also lose the matching employer contribution. Years ago Transocean tried to tell employees they came out ahead by not having to pay FICA, more take home pay! In actuality TO was saving themselves from having to pay the 7.5% match while the employee was reducing their retirement benefit.
Same at Global Marine (and then GlobalSantafe) when I worked there.