Grammar: Grammar is something that bothers me on a daily basis. Your grammar is good, but not very tight. For instance, I thought that your first sentence could have been removed completely, which would also eliminate a shift of person (plural first to third).

You break into first singular first person, out of nowhere, really. "Obviously insinuating that neither task will be cheap, but I suggest we clean up one mess before we create another." First of all, this sentence should be part of the previous sentence because the "obviously" obviously modifies it, not your sentence. Second of all, I think this needs to be either at the top or the bottom, with your thesis or conclusion.

You also need to pay attention to your i-t-ses. It's = it is. Its = possessive pronoun. Its' = not a word.

I'm going to use the same example to explain two things. Some of your diction is empty or begs for further definition; there are some grammatical conventions that you didn't follow. For instance, I would pluralize manatee, add another comma and hyphenate environmentally-friendly; and add the stuff in parenthesis. "This canal, and the manatees (in it), rely on the bay's natural flushing cycle to dispense(discharge) the (process' waste) discharged solution in an environmentally-friendly manor."

You could use an edit pass for conciseness. "The Tampa plant is going to," could be, "The Tampa plant will." "our ultimate goal, complete water sustainability, using all we take and wasting none," becomes, "our ultimate goal: sustainability." Incomplete sustainability? (One day before school, I was watching "Judge Alex" (because I get a kick out of it) and he asked a defendant for his probable chance at a negative result on a drug test, to which the defendant said, "about 85-90%." The Judge replied, "you don't just do 85-90% drugs. You either do or you don't." This is one of those situations.)

Investigation: You have a good ratio of fact to statistic. Seriously, though, you did your homework and it shows. I wanted to know more about the manatees and the desal plant's proximity to the power plant.

Purpose: The purpose of any definitional practice is to form the ideas of others. You have given a brief history of water-shortage concerns in our geographic area, explained how desalination works and slipped in your own opinion. Could you have defined the same problem, in the same light, without placing yourself in the middle of it all? I think that you could construct a more subversive, effective argument by focusing on sentence level revisions.

Improvement: I almost gave this point back to you. I reviewed your changes, but it didn't look like you wrote much more or changed much except for the order. Basically, this point is deducted because the errors that I noticed are consistent throughout. It would be great if you revised and reclaimed some points.