英语翻译中文：在过去,主要的解决方案有海水淡化或建造大坝运河.可是,这两种解决方案都存在着一些问题和不足.英文：Major solutions in the past have desalination and the building of dams and canals.However there are many problems and shortcomings in those solutions.

in the past, major two solutions are desalinizing seawater and buiding dams and canals.(或者这里是canal dams依全文而定). However, either has its problems and disadvantages.

总的来讲您的翻译没错的，如果需要稍微改动更好的就是：
Major solutions in the past are desalination and building of dams and canals. However there are many problems and shortcomings in those solutions.
或者：Desalination and building of dams and canals are the main solutions in the past.Howere,there still exist some problems and shortcoming.

There are desalination and the building of dams and canals for major solutions in the past. However, both solutions still exist many problems and shortcomings.

Major solutions in the past were desalination and the building of dams and canals. However, there are some problems and shortcomings in those solutions.
我只是修改了一下语法。

Major solutions in the past are desalination and the building of dams and canals.
However,there are many problems and shortcomings for those solutions.

Major solutions in the past are desalination and building a dam or canal. But problems and shortcomings exist in both of the solutions.

我认为
1.用are 不用have
2.英语中状语习惯放在后面，in the past 的位置调整一下
3.原句是一些问题，你用many就是许多问题了，有点不忠实于原文。