One of the coolest apps I've seen if you aren't on a boat with a depth / fish finder is called "Deeper". It let's you "cast" out a bobber-like device that emits sonar and sends fish, depth, terrain, etc imagery back to your phone. Really cool for kayaking, paddle boarding, etc. Check it out here.
I also saw an ad for FishAngler in South Florida recently - Looked it up - Looks cool (social network for fishermen, logging, species info, etc), but not sure if it's any good.