Because water is a mass noun and in French mass nouns normally (there’s some edge cases) use the partitive instead of the indefinite article, and the contracted forms don’t occur when the regular part of the article is elided, so… de l’ (as opposed to du).

De is used because sea water hasn’t been previously mentioned. The definite article would imply the sea water is one of several known things.

Here is a case where l’eau alone could have been used:

Il y avait de l’eau de mer et du sable. L’eau de mer a été utilisée pour empêcher la fusion des réacteurs.

The indefinite article can’t be used (une eau de mer a été utilisée…) because there is just one kind of sea water around Fukushima and it is uncountable. You can say un vin, une bière but not une eau de mer.

De l’eau de mer means “some unspecified amount of sea water”. Unless in English, we can’t use “sea water” alone, i.e. eau de mer without article, to mean it.