This has been a topic of discussion on these boards many, many times. The common belief is that the ride was way too mechanical for it's own good.
It ran on diesel (smelly, nasty, polluting), loaded and unloaded very slowly, didn't hold many guests at one time creating long, long lines, required a large contingency of CM's to make it operate and although it was a unique ride it was also laughably amatuerish with its brightly colored plastic fish, held in place by plastic line. I am sure it was starting to get very expensive to maintain as well. I think it was a classic but it was to big to be continuiously done.
I'm sure others will add more to this. I can't imagine it having to do with the "walls" because the lagoon is still there 8 plus years later. Who knows??