Dan, your best bet to stay 100% legal is to buy a radio like an older Icom 710 or a new 802(?) which is manufactured and approved for both services. It will have drawbacks, but so will anything else.
As long as your radio is legal for use in your home country, it will be legal around the world by reciprocal treaties via the ITU. (Of course, if you are visiting "Red" China you'll need a permit, because bringing in an unlicensed ham radio is quite literally an espionage conviction there.)
Marine HF radios are "channelized" so you can dial up a channel number. Ham radios use tuning dials/pads, because hams don't use numbered channels. A case can be made for having two radios--unless the new Icom gives you both options.
As xort mentions, it is rare for anyone to get busted for offshore use of anything--as long as there are no complaints. INshore...that's something else, since marine radios are much 'cleaner' than ham radios with regard to spurious emissions, and have a more limited audio range--so a good ear can quite literally tell what you are using by the sound of it, and the frequncy spatter.
I'm sure you'll find older threads on this subject here, if not on other web and ham forums.
If you are not technically inclined, and don't want to invest in one of the dual-purposed ICOMs (I don't know if any other vendor makes similar radios), I'd suggest spending money on the marine HF, and then adding a lower cost used ham radio form a reputable source (i.e. one of the stores) to supplement it. That way you are fully legal, fully redundant (if you use two antennas) as well. And there are plenty of HF ham radios, used, at reasonable prices.