As an outdoors man, there are not many apps for us. I have thought a very simple app is one that gives me up to date info on my local waters that I fish. I can go online and look at the river data and get info that I want, but it would be awesome to have this right on the phone. Info I am most concerned with is for the Mississippi River. Data such as flow, water temp, pool stage etc is very useful and updated daily. There are also sites that give the same info for lakes and other rivers.
Here are two sites I regularly use.
River water is a quick look at the pools of the river I fish.
Lower Mississippi River Basin Forecast - River Watch
River Data gives me a little more detailed info and allows me to see history for trends that are very important to fishermen.
USAED-St Paul District Projects
Obviously feedback would be important to the dev so that things can be added to the app in future updates. During the winter months, ice thickness and water clarity could be items that users can add to the app to share to other fishermen.
Good idea for the dev that is passionate about it. If I had the skill, I would develop this myself.