Ni hao, to say : assert , communicate ,comment , express, mention , tell , utter . 说 , 发言, 默念 , 该说的话, 发言次序 问好 = to say hello to ,to send one's regards to. to speak : express, talk 动 说; 讲;交谈 ;演说;传达; 请愿. speak to 同...谈话, 对...讲话 speak well for 证明…很好 While the 2 words seem so close in meaning that they could be synonyms you can not use them in the same context nor while applying the same sentence structure. Speaking is more of a context conveying a message and being communicated .It is mostly about a subject not just a single word, it includes a wider scope of info being passed ranging from view words and a subject to a language, while "to say" is more limited to a word or a phrase as in your example "how to say water in English?" but you could " speak about the lack of water resources ". So saying something is more confined within the limits of an instant verbal speech being communicated while speaking is more extended to a subject with a relatively increased scope of speech and time of delivery.

http://www.thefreedictionary.com/speak http://www.thefreedictionary.com/say

I'll try to explain the difference in English, since I can not do it in Chinese. You speak with somebody, you speak a language, you speak about something, but you say something. Examples: How do you say "water" in Spanish? I'm going to speak about water and other removable resources. May I say something? I need to speak with you ! I know how to say "good morning" in Chinese I do speak Chinese. Say something to me ! Do you want to speak with me? Is the difference clear for you?

here are two examples: Can you say that again? Do you speak French?