Cedric, I can't really follow your code or your explanation, but I would think you want something like this:
ice = readtimetable("Ice concentration data.csv")
opts = detectImportOptions("15330-01-JAN-1970_slev.csv",Delimiter=',',NumHeaderLines=7);
opts = setvaropts(opts,"Obs_date",Type="datetime",InputFormat="uuuu/MM/dd");
water = readtimetable("15330-01-JAN-1970_slev.csv",opts)
Throw away water data outside the range of the ice data.
water = water(timerange(min(ice.DateTime),max(ice.DateTime)),:);
Fill in the ice data with missing values and plot.
ice = retime(ice,water.Obs_date,"fillwithmissing");
plot(ice.Conc_Ice_Avg,water.SLEV_metres_,'o')
If your goal is to overlay all the years on top of each other, thatneeds a few more steps, but your description doesn't seem to say say that.
