its mayim she'ein lahem sof. why would she not be an aguna?
My understanding as well
"The halakhah, while striving to be lenient as possible in the method of proving the husband's death, imposes strict requirements concerning the nature of the evidence with regard to the husband's death, lest a woman still married may thus be permitted to marry another man (Maim. ibid. 15: Sh. Ar., EH 17:29). The identity must be established of the person whose death it is sought to determine and there exist most detailed rules in order to establish it with the maximum amount of certainty under the circumstances. Thus evidence as to circumstances from which death would be likely to result in a majority of cases is not considered as sufficient proof of death itself since it may be merely the opinion of the witness that the husband is dead, but not testimony as to the fact of death. Hence, the wife will not be permitted to remarry on the strength of evidence to the effect that her husband was seen to fall into the sea and drown in "water having no end" (i.e., where one can see only the sea but not its surroundings) when his death was not actually seen to have taken place, since he may have been rescued. If, however, the witness testifies that he was later present at the funeral of the husband or some other clear evidence of death, for example, that an identifiable limb was found at the place of drowning, it is accepted as evidence of death. On the other hand the death of the husband will be accepted as having been sufficiently proved and the agunah will be permitted to remarry on the strength thereof if there is evidence that he drowned in water "having an end" (i.e., that one can see its surroundings); and the witness stayed long enough at the scene "for the victim's life to depart," without seeing him rise to the surface (Yev. 120–121; Maim. ibid. 15–27; Sh. Ar., EH 17–42, esp. 32)."
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