The initial impact of a tsunami is dramatic, but the long-term effects of such flooding can be just as devastating for survivors.
Lack Of Fresh Water
The first long-term problem facing tsunami survivors and disaster responders is procuring water. A huge amount of salt water has swept over the freshwater delivery system that modern society has installed. Furthermore, septic tanks and sewage systems have often burst open. Chemicals and pesticides can render water undrinkable and unsafe for any kind of contact. The initial deliver of bottled water and an ongoing system of scrupulous water testing is a must for the affected areas.
Gathering people into shelters provides immediate and necessary relief, but any large crowd of people together means that any illness can spread fast. Providing basic things like soap and vaccinations for young children helps, but most disaster relief organizations also need to monitor health conditions in disaster centres and provide extra care for elderly or immuno-compromised victims.
Tainted Water Supplies
Long-term relief is needed to help rebuild homes, schools, hospitals and businesses in the affected areas, but again, it all comes down to water. Especially large tsunamis dump an enormous amount of sea water on land; this water can poison farmland and eventually seep down into groundwater supplies along with chemicals or pollutants. Even months after a tsunami, when homes are rebuilt and people move back into an area, water supplies need to be monitored carefully make sure they aren't compromised.
All of this need for water quality control, scrupulous healthcare, and tons of fresh water comes at a time when a community has been economically devastated, so a great deal of help is needed. The Red Cross has set up a Japanese tsunami relief page where you can donate money, as have many other world health organizations.