There are about 3 tonnes of uranium in every cubic kilometre of seawater, but in the general circulation there is no mechanism to concentrate the uranium, except perhaps low-level adsorption on manganese nodules in some parts of the sea bed. To concentrate the uranium you need some mechanism to transport it, and some other mechanism to make it precipitate. Both are pH-redox controlled, requiring substantial reducing conditions. The nearest plausible mechanism I can think of is a high concentration of uranium in groundwater, such as in the Aktau province of Kazakhstan, adjacent to the Caspian Sea. At present the Caspian is too oxygenated, and is likely to remain that way because of inflow from the Volga, but if climate change alters the rainfall-runoff relationship then that inflow to the Caspian would be reduced. Conceivably, it wouldn't take too much more pollution and reduced river inflow to make the deeper parts of the northern Caspian more reducing. Then maybe you could get uranium deposition at the groundwater-lakewater interface.