No predictions loaded for 1990. This site serves NOAA predictions from its own store, and this period has not been fetched for Santa Isabel yet. Nothing is estimated or interpolated in its place. NOAA's own page for the station has the official table: tidesandcurrents.noaa.gov.
tidealmanac.site/us/pr/santa-isabel-9756639/1990
Santa Isabel, PR tide table 1990 (printable) · Tide Almanac