Sea walls, of course, may help protect against tsunami waves, provided that they are not undercut, but do nothing about keeping contaminated water from seeping into the substrate or flowing to the sea.

As nuke workers wait, tainted water climbs
Japan Times, Kazuaki Nagata, May 28, 2011.

Tepco must find a safe place to store the thousands of tons of radioactive water being generated at the Fukushima No. 1 plant before it seeps into the ground or finds its way to the sea.
Compounding the problem are the reactors, which are believed to be riddled with cracks and damaged pipes which allow the water used to cool the reactors to escape. Plugging the leaks is difficult because high radiation levels prevent workers from dealing with the problem.
As the amount of water increases, the problem becomes worse.
The water is interferring with workers attempts to deal with the reactors.
Basements of all six reactors have been flooded with 100,000 tons of radioactive water.
Cracked utility pits were found to be draining into the sea after being filled by trenches linked to the turbine buildings.
Tepco must wait for a water treatment facility.

Most of Tepco's hopes are pinned on a treatment facility being set up by Areva SA (France). The facility removes radioactive substances from water.

Reading further, problems are compounding problems.
The treatment must continue for "some years" so the company must find some other way to secure the reactors.

True, the reactor problems have disappeared from news broadcasts and other media.