The fish is seeking water - it just has an intellectual understanding of water but doesn't feel it yet. It is told that all experience is water but it cannot understand, because it is looking for something different, for something to change. It isn't about some new experience - it is pointing out that what IS, is already that.
In the same way, in spirituality we are seeking something new, something different. It is this requirement or condition we assert upon reality which makes us overlook the obvious simplicity of this message.
The problem, if we can call it a problem, is ignorance of what we are. The solution is knowledge - self-knowledge. There are many kinds of knowledge. If I want to know about cooking, I might watch a cooking show or buy a cookbook. If I want to learn how to program a computer I might take a class. However if my desire is to know myself what training can I take? What book can I read? What show can I watch to discover my Self?
Therefore the means of Self-knowledge is spirituality - there are many forms of it - in this case here we speak of Vedanta. Vedanta is a means of knowing your Self - finding out what you are. And to find out what you are we must first know what we THINK we are. Therefore we dive into our current ideas - we find we have never really examined these ideas in any depth.
We take ourselves to be some organism, some body which was created from the cells of the mother and the father. From the combination of these cells this body, this organism took shape, grew, gained capacity through it's various organs to function, breathe, heartbeat, heal, think, etc. It is a functioning organism and eventually it has mental capacity - it learns and can gather beliefs. Ultimately there is the capacity we call Consciousness.
Our idea is that we did not exist prior to this creation, prior to this conception. We simply did not exist. Then at some point we came into existence. One minute we did not exist and then the next we did - this is very deeply ingrained. However the main point is that we popped into existence where we did not exist before. Then we exist for a while, that organism grows and goes from a fetus to old age, then when these capacities stop, when the breathing and heartbeating stop, we stop. We STOP. We cease to exist where we once existed. One second we were there and then we are gone. We're GONE.
So think about that idea - think about this concept you have of what you are. Consider it deeply - really realize what it is that you take yourself to be. And then compare it to your goal of spirituality - which is to find oneness or whatever you want to call it. You take yourself to be something which is temporary and independent, yet you yearn for oneness. Your very existence as you take it to be contradicts what you hope spirituality is all about.
Consider deeply this idea of your birth and your eventual death. Are you something which comes and goes? Can existence really come and go? What is it that comes and goes? Is it existence or is it just forms? Isn't the body-mind just a form, a pattern, an organism, a particular arrangement of atoms or particles? Just because they are shaped as a body for a while doesn't mean that YOU are born and then die. Just because energy patterns in a particular way for a while doesn't mean that YOU are temporary.
Intelligence is the basis for everything you know. It is all around you. In fact, like the fish, you are surrounded by it only you focus on the appearance of it instead of the reality of it. In focusing on the appearance you take the fleeting nature of appearance as the fleeting nature of existence or Intelligence. Wave isn't something new, something born, something which didn't exist before and then comes into existence, later to die. Wave is an appearance OF ocean - the only thing which comes and goes about it is the appearance of wave, the activity of ocean, the expression of ocean.
In the same way, this body-mind is an expression of that Intelligence, of that Existence - you aren't a new and temporary existence but that Intelligence itself - coming to know itself because OF this appearance, because OF this capacity we call consciousness or knowing or awareness. You have mistaken yourself for the appearance - you have identified with a fleeting form and overlooked your Presence as THAT which IS.
Tat Tvam Asi - You Are THAT. This is the meaning we should discover.