I dont know the memory-requirements of your game, nor the XBox Limitations.
I just want to point out, (If you knew that the game will also run on the X-Box at the beginning)
that before stuffing the levels and creating the content, there should be a strict guideline
for how big the content is allowed to grow.

The optimal choice is to use the limits out to the max, so that it still works, and
not overshoot the limits.
I also dont know, how hard it is to calculate the limits beforehand.
But the easiest way to to this, is to create a test-prototype and stuffing more and
more models, sounds, BSP and slow-loopfuntions into it, to see what the XBox can handle.
(the models can be just some random-highpoly stuff)
With this knowledge, you can estimate how much the game can handle good.
Also the XBox is a fixed system, with little surprises (in contrast to
the many different PC configs)

Given that other very nice looking games can be realized with the XBox,
I have to say, that a Fishing game can be realized for shure too.