After doing a titration my concentration of Magnesium ions is slightly higher than the literature value. Why could this be?
Also my concentration of Calcium ions is slightly less than the literature value so also why could this be too?

Margin of Experimental error

See above and note that sea water varies depending on the season and location. Sea of Galilee is vastly different then the Arctic Ocean. What did you do during titration and how did you determine the Mg and Ca levels? Did you use the EDTA method? Maybe you are color inhibited since that is how the ions are determined.