how do I fix following errors? help please

Assets/Standard Assets/Scripts/Performance/PerformanceTweak.js(2,13): BCE0018: The name 'IslandWater' does not denote a valid type ('not found'). Did you mean 'System.Runtime.Remoting.Channels.IClientChannelSink'?

Assets/Standard Assets/Scripts/UnderwaterEffects.js(15,20): BCE0018: The name 'GlowEffect' does not denote a valid type ('not found').

Assets/Standard Assets/Scripts/UnderwaterEffects.js(16,20): BCE0018: The name 'BlurEffect' does not denote a valid type ('not found').

Assets/Standard Assets/Standard Assets/Camera Scripts/Mouse to Look.cs(18,14): error CS0101: The namespace `global::' already contains a definition for`MouseLook'

I assume these are the compiling errors that wont let me test it. can some one tell me what the mean and how to fix it with as much details and step by step instructions as you please.

It means that for whatever reason Unity cannot find the script when you try to access it. More than likely, it means that you are compiling scripts in the wrong order. This page is your friend. If that doesn't help, then you might want to make sure that you entered the name of the class correctly.

CS0101: you have multiple classes (scripts) with the same name. Change the name of one of them to solve the problem.