It has to be dissolved in an acidic solution or it will precipitate out as ferric hydroxide at the normal surface pH of seawater.
It (the iron) has to be chelated. There is so much bicarbonate in the ocean that any added solution pretty quickly assumes the pH of 8.2.
Our blood has a carbonate-bicarbonate buffer (not coincidentally, at pH 8.2), and we have hemoglobin in our blood as a chelating agent so the iron (needed in oxygen transfer) doesn't precipitate out.