Why does the Marshall Islands make it (with 98%, which means they must be counting the ocean between the islands), and yet countries like Micronesia, the Solomon Islands, and Cape Verde do not. They would have to be using an inconsistent means of measuring. Aside from a few like Canada, Nicaragua, The Gambia and Malawi which can be surmised through some logical deduction, the rest of this is almost an impossible crap shoot.
Just took a quick look and didn't see any enclosed atolls. In other words, every atoll I saw had at least some non-reef access to the sea. That said, even a submerged reef presumably would rule a lagoon out, wouldn't it? Of course, if I missed just one atoll, and if the entire lagoon would then be counted as inland water, that might skew the numbers hugely. My guess, though, is that this is just a difference in definition between jurisdictions. That, by the way, must be the case for Guinea-Bissau, Eritrea, and Liberia as well.
Before the independence of South Sudan, the combined inland surface water of Sudan was about 130 000 sq kilometers, and the percentage of inland water of the total area was 5.18%. Due to the war, figures are not available of the two countries now. If we just knew their figures, at least another of them would be ranked high on this quiz.
Hard, but fun