A two-phase or hybrid immersive cooling system is converted to a one-phase immersive cooling system, by (1) modifying a structure of the two-phase immersive cooling system such that a cooled portion of a substitute operating fluid flows into the operating fluid reservoir from a direction other than from above the reservoir, and (2) using a high boiling point composition as the substitute operating fluid. In some embodiments the tube condensers of the two-phase or hybrid immersive cooling system are retained, and in other embodiments at least portions of the regions previously occupied by tube condensers are used for heat exchangers, and optionally fluid pumps. In still other embodiments at least portions of the regions previously occupied by tube condensers are used for hot fluid holding tanks.