All spawns are visible and accessible for all players. Incense is the only exception. Tell me how your rod could work without violating this rule.
So you run around with it near water, and it spawns pokemon only for you? Or it uses only the pokemon that are already there? In the second case, it is strictly worse than pokemon go plus.
And how would you sell it to anyone who does not have many rivers or lakes in his home town? How would you avoid screwing people living in rural areas with no river or lake nearby? How would you avoid it looking so incredibly silly that people are embarassed to use it?